Some thoughts on Some thoughts on Gief’s aerials:
J.FP: Good in fireball wars. Smack them in the head if you anticipate a fireball and jump it. Even if you land on a fireball it tends to be a trade in your favour. This is go to for range as you’ve probably figured out, but it’s worth pointing out that this it’s not helpful at close range against people with fast walk speed since if they decided to pressure with an advancing poke or move then they can just go straight under gief and get a throw/combo. Other go to aerials don’t have this problem as much.
J.MK: Your deep air to ground jump-in. Not always guaranteed, but it has the best chance to beat/trade with crouching normals. Can be used well as a tick since if you’re in range for J.mk you generally land in jab spd range. Will beat horizontal and vertical blanka balls clean most of the time when it is out (although vert ball comes out so fast it’s generally not a great thing to try too much).
Body Splash: Not great priority against grounded AA normals, best for crossup, but has a nice wide hitbox to threaten with when used as a non-crossup jump-in at close range. Generally works best when timed to hit as soon as it comes out so you don’t widen your hittable box for longer than is necessary on the way down to hit them.
Double knees: good tick, good crossup to stuff jumps and possibly some DPs too. Shifts trajectory and makes you unpredictable. It’s biggest problem is the lack of hit or block stun because generally since anything that is not a command throw is easier to punish/beat/trade/reversal. It’s good for a fast tick that is hard to react to, but generally for cross-ups I tend to prefer splash since it gives you much more hit/block-stun to work with. More of a mixup than a go-to aerial imo.
S.MP: Good Air to air normal. Nice range and speed (neutral j.mp also has a nice bit of range, but I tend to use headbutt since it’s a lot more threatening to the opponent and a smart one will generally be pretty scared of it)
J.HK: I think this is less favoured since it has less range compared to J.FP and doesn’t hit as deep as J.MK, but it works well as an Air to Air (like when they decide to jump away from you at close range). Not bad for close range jumping I suppose and it can be used to trade easier in some situations other aerials (Claw’s flip-kick springs to mind) but the previously mentioned details about J.FP and J.MK illustrate why it is less popular. Perhaps it’s worth more at close range for extra hit/block stun.
Headbutt: Not a jump-in, but I’m sure all gief players realise how dangerous this move is if you know the right situations to apply it in.
The rest of gief’s aerials aren’t ones I see used at all really. Some may be good for certain specific scenarios but the other aerials I’ve mentioned here are probably just as good/better with a wider range of possible applications