Unfortunately, Gloom is one of those Pokémon that does not evolve by level. If you want Gloom to evolve into Vileplume, you will need to find a Leaf Stone. If you want to make it evolve into Bellossom (which appeared in the second generation (Gold/Silver)) you will need to find a Sun Stone.
Also, this is not specific to FireRed, evolution factors are the same through all the different games (though certain pokemon, like Tangela, can only evolve in later versions of the series).
For more information, try this site: http://www.serebii.net/pokedex-dp/044.shtml
It evolves in Gloom at level 21.
Oddish evolves into Gloom with a leaf stone, or at level 21 Gloom evolves: Into Vileplume, using a leaf stone on Gloom. Into Bellosom, using a sun stone on Gloom. ^_^
Use a Sun Stone on Gloom to evolve it into Bellossom
It evolves into Gloom starting at Level 21
Gloom and Sunkern.
Gloom can evolve to Bellosom with sun stone....
gloom into vileplume.
Personally i would say as soon as you get gloom evolve it.
any level you want to evolve it att
it doesnt
Weepinbell, Gloom and Exeggcute.
gloom