theres two ways to evolve gloom.one way is to use a sun stone and it evolves into Bellosem(i spelled it wrong i know)or you can use a leaf stone to evolve it into to veleplume
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Use a Sun Stone on Gloom to evolve it into Bellossom
gloom can evolve into bellosom and sunkern can evolve into sunfloria.
You have to get an Oddish first, then evolve it into Gloom. After that you give Gloom a Leaf Stone and he will evolve into Vileplume.
Personally i would say as soon as you get gloom evolve it.
It evolves into Gloom starting at Level 21