Gloom evolves with the use of a Leaf/Sun Stone, not at any specific level.
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Use a Sun Stone on Gloom to evolve it into Bellossom
It evolves into Gloom starting at Level 21
Personally i would say as soon as you get gloom evolve it.
Oddish evolves into Gloom at level 21. Gloom can evolve into Vileplume via Leafstone or Bellossom via Sunstone
In SoulSilver, Oddish will evolve into Gloom at Level 21 however Gloom can evolve 2 different ways, you can use a Leaf Stone on it in order for it to evolve into Vileplume or you can use a Sun Stone on it in order for it to evolve into Bellossom.