In a nutshell
Gohan is a very defensive character capable of effectively countering every method of attack his opponents try. He is a charge character, and as such, is perhaps the only character who is at his best when down-backing. He has the best anti-air in the game in the form of a flash kick, dangerous rushing attacks and multiple good guard counter options. The trade off is that he is generally bad when he is forced into action, having a below average hop speed, mediocre mixup game and no good options against heavy zoning. For Gohan, defense is the best offense; space yourself at midrange so that you can react to any attempt your opponents make to mount an offense. Good choice for players who like being the brick wall.
Close st.Y - Quick jaw-level lefty. One of the better close standing normals in the games due to its good speed and range. Fulfills its purpose as a combo tool.
Close st.B - Thrusting knee attack. Can't be special cancelled. A frame slower than close st.Y, but deals 2 more damage. Not as highly recommended as close st.Y since it pushes the opponent so much further back on hit.
Far st.Y - Straight forward clock to the face. Pretty fast for a far standing normal, but still not very useful since everything this move could do, another move you have can do better.
cr.Y - Gohan rises up with an uppercut punch. Fastest cr.Y in the game. Very deceptive active frames; the active frames end when Gohan's fist stops moving. The final, lingering part at the end has no hitbox. This kinda limits its use as a viable anti-air, but it is still useful as a quick way to stuff dash up throw attempts and close range hops if you don't have charge.
cr.B - Gohan sweeps the leg like a true scumbag. Hits low. Knocks down. Special cancellable but not normal cancellable. Not a bad low, but the fact that it forces knockdown limits its usefulness because it prevents any further combo potential. Still, good poke to use when you are downbacking, since the opponent certainly shouldn't be hopping at you when you have charge.
Y - Gohan slides in with a forward moving cr.Y. Launches on hit. Can't be normal or special cancelled. Same active hitbox properties as cr.Y, but 4 frames slower in exchange for some fast forward momentum. Makeshift anti-air and very strong surprise attack to suddenly pull out on someone when they think you're just turtling, although it is punishable on block so do be careful. On hit, can follow up with up to 3 air juggles. Take care when using this move while downbacking; it will come out as Bakuretsu Punch if you're not paying attention.
j.Y - Just a jumping punch. Can't be special cancelled. Not very impressive, but it does have a slightly better air-to-air hitbox than j.B. The other use for this is as a tick throw tool since it is very easy to whiff it against standing opponents. Hop forwards, press j.Y early, whiff, land, throw.
j.B - Incoming outstretched leg. Special cancellable. 2 frames faster than j.Y. Very strong air-to-ground due to its fast start up and great diagonal reach. Go-to jumping normal.
Bakuretsu Punch - charge , Y - Gohan dashes forward with a series of four quick punches. Knocks down. This move does very good damage on its own, but for combos, you will almost always get more damage by comboing into launcher. Not good to use in blockstrings either; it's just begging to be guard countered. The best use of this move is as a guard counter special, since it comes out fast and is one of the most damaging specials in the game.
Knee Slasher - charge , B - Gohan slides forward on one foot with his knee in the air, connecting with a knee-roundhouse combo. Knocks down. The slide effectively goes fullscreen and acts as a forward moving hitbox for Gohan. The problem with this move is that if it hits an airborne opponent, they will flip backwards, land on their feet and then punish you. The safest place to use it is on the ground at midrange, where it is still unsafe on block in addition to being slower on startup than most far standing normals. Not worth it.
Zanku Kyaku - charge , B - Gohan's flash kick. Fully invincible on startup. An incredibly strong reversal and flawless anti-air, this move is the reason Gohan is untouchable on defense. The simple command allows you to easily input this move at any time during blockstun as long as you are crouch blocking, and unless you're facing Cell or Vegito, the opponent has no way to force you to lose your charge other than jumping at you, which obviously wont work. Using this move well is the key to playing Gohan.
Bukuuh Kyaku - B while in the air - Gohan's divekick. Slow, but completely unpunishable on block, giving it more utility than other divekicks. Feel free to throw it out, just be careful of being anti-aired or stuffed during its slow startup.
Triple Fireball - A - Fires 3 generic fireballs in fast succession. Costs 5 health to perform. Not a bad move to use to beat out other level 1 projectiles, although the recovery leaves you quite open if they decide to jump, so best to avoid using at midscreen. Level 2 projectiles and above will eat through all 3 fireballs.
Masenko - A - Gohan throws a powerful energy blast diagonally up-forward. Generic anti-air projectile attack. Costs 10 health to perform. Does its job for the most part, too slow to use on reaction to hops but is still useful as a punish for full jumps. Gohan has much better options for anti-airing at closer ranges.
Kamehameha - A - KA! ME! HA! ME! HAAAAAAAAAAAAA! Standard chargeable projectile attack that most characters have in some form. Costs 10 health to perform. Hold down A while performing in order to charge it up to a maximum of 3 power values. No charge will quickly release a level 2 projectile. Medium charge will release a level 3 projectile. Maximum charge will release a highly damaging level 4 projectile.
Desperation Attack: Gekiretsu Rush - B - Gohan charges forward with a devastating combo. Most damaging desperation attack in the game at 186 base damage. j.B -> close st.B -> cr.Y -> Desperation does 243 base damage. Definitely a strong combo tool when it's available, although not very useful for anything else.
Gohan is a charge character. Make sure you know your charge timing and adjust accordingly.
Gohan is fairly decent at mid range, especially when he has his Zanku Kyaku (flash kick) charged. With a flash kick charged and ready to go, you can react to mostly any air approach with it, and ground approaches can be beaten with his sweep. If you don't have a charge ready then you'll have to be more proactive. He has a short forward hop and his j.B is really good, so at closer ranges he can compete in hop wars. His divekick, while slow, is completely unpunishable on block, so as long as it doesn't get stuffed on startup you're safe. Y is a nice surprise attack to pull out occasionally when you are downbacking, but it's punishable on block so don't go crazy. You can also regenerate health by holding B+Y while still maintaining your charge, so use that to make the opponent try to rush you and then stop and punish them. Gohan can't do anything at long range, so if your opponent is trying to zone you out you'll have to take it to them. Your options at this range are to concede and start hopping and jumping at them, regenerate some health with B+Y, or 3D attack through any slow or obvious projectiles.
Gohan's weakest point, he doesn't have a whole lot to threaten with on offense. His hopping j.B is good, so the standard hop pressure should work just fine. His j.Y also whiffs against grounded opponents when done early, so you can use that to empty jump throw. Otherwise, you can plant yourself a few spaces away from them and just react to any attempt they make to escape the corner. Flashkick will beat all hops and jumps. His sweep is his only low; it's good but he doesn't get combos off of it. Try to avoid using his special moves in blockstrings, almost all of them are multihitting, making them very easy to guard counter and punish.
Gohan's strongest point. Flashkick is amazing, it beats all jump-ins and when used as a guard counter it will punish almost anything on block. When combined with reversal throws and reversal hops, Gohan is really hard to open up. Just watch out for throws, the opponent will probably go for a lot of them. If you see the opponent forward dash near you, chances are you can hit Y to punish it.