With hi-res sprites from Pokémon Type: Wild, you'd think Gengar would play like it does in said game; though you wouldn't be completely wrong, it's not exactly accurate either. An interesting feature Gengar has is the ability to poison its opponent with two of its moves, but the effect is quite glitchy and doesn't always go away.
Gengar is a six-button character, though only five of those are used for attacking, as (C) activates a Dodge when used in conjunction with either the left or right directional buttons. According to the character's ReadMe, (X) is Punch, (Z) is Kick, (B) is Lick, (Y) is Slam 1 and (A) is Slam 2. Like many other characters that were inspired by Pokémon Type: Wild, Gengar has the option to select one of three assists at the start of the round, or fill up its Power meter by 1000 (the increase in Power caps at 1000, so it won't go any higher if the option is selected while Gengar's Power is higher than 0); the assists are Ekans, Medicham and Haunter.
Gengar has reasonable combo strings, but such combos require very precise timings to pull off, making it fairly hard to use. Gengar does have quite a few infinites, though like its actual combos, the time window to pull them off is fortunately very small, with the exception of one particular infinite that can be executed by alternating between (X) and (Y) while crouching; this infinite is incredibly easy to do and takes out the entirety of the opponent's Life within seconds.
Gengar does not have a custom A.I., meaning it uses the engine default. Because the default A.I. has problems with moves that can be held down, it has a habit of using Advance Guard until its Power has been completely drained.