There are two ways to evolve a Gloom, you can either evolve it into Vileplume or you can evolve it into Bellossom. To get Vileplume all you need to do is use a leaf stone on it. Likewise to get Bellossom you have to use a stone on Gloom, but this time it is a Sun Stone.
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Evolve an Oddish to Gloom then use a Sun Stone to further evolve it to Bellossom. You must have the National Pokedex upgrade in order to evolve it otherwise nothing will happen.
Gloom doesn't evolve into Bellossom by leveling up. Use a Leaf Stone and TADA! A Bellossom! : )
Raise an oddish to evolve to gloom, or catch a gloom in the safari zone, then evolve it with a sun stone. You should get bellossom.
In all of the Pokemon games Oddish evolves to Gloom when it is level 21, then you can evolve it into Bellossom with the sun stone or Vileplume with the leaf stone.
Use a Sun Stone on Gloom to evolve it into Bellossom