Technically, Gastly had Hypnosis at level 1 and by the time you catch it the move has been overwritten by moves Gastly learns at higher levels . So, you need to obtain a Heart Scale in the underground and take it to someone in Pastoria City, who will then be able to teach Haunter Hypnosis.
It Learn Hypnosis At Level 16 Must Catch Level 16
Do you mean "Haunter"? Haunter should already know Hypnosis. If it doesn't, use a Heart Scale. This is not a wise comment, this is the answer.
In Pokémon Crystal, Haunter learns Hypnosis at level 40. This move allows Haunter to put opponents to sleep, which can be strategically useful in battles. Players can also use TMs to teach Haunter other moves if they prefer different strategies.
level 45 in fire red leaf green
At level 200
haunter learns it at level 34
Slowpoke doesn't learn hypnosis
Wobbuffet cannot learn Hypnosis.
Gastly > lvl 25 > Haunter > trade evolution > Gengar
Snorlax does not learn hypnosis.
In Pokémon FireRed, Gastly evolves into Haunter at Level 25.
There is no TM for Hypnosis in any of the versions of the game. Unfortunately, the only way to learn hypnosis is through breeding or your Pokemon leveling up. Level up haunter and it might get hypnosis.