Unlike most stone-based evolutions, Vileplume and Bellossom actually learn a few moves at levels other than 1. Vileplume learns Petal Blizzard, Petal Dance, and Solar Beam (at Levels 50, 53, and 65) while Bellossom gets Magical Leaf, Petal Blizzard, and Leaf Storm (Levels 23, 50, and 53, respectively). This means you can evolve Gloom earlier than what's typical for stone evolutions, as it can still learn strong moves later.
Oddish is first available on Route 6, just a short distance away from one of Gloom's evolutionary stones: the first guaranteed Leaf Stone can be found on Route 8. At this point you can't use it (unless you're significantly overlevelled), so it's best to focus on evolving Oddish first. The first guaranteed Sun Stone you can get is located in Shalour City, and it's around here that Oddish should be evolving. This means you're presented with both of Gloom's evolutionary options almost immediately.
By this point in the game, your Gloom's most valuable moves learnt by level-up will probably be some of:
- Poison Powder
- Stun Spore
- Sleep Powder
- Mega Drain
- Acid
The next good move in Gloom's moveset is Giga Drain at Level 47, but it's fairly inefficient to use a Gloom until that point in the game. Therefore, it's best to evolve your Gloom immediately once it learns Mega Drain if you're choosing to use Vileplume, and a few levels before that if you want Bellossom.
The reason for this is that Vileplume doesn't learn another attacking Grass move via level up until Level 50 (Petal Blizzard), so it's important to have the strongest one you can. Bellossom gets Magical Leaf at Level 23, which is just a bit better than Mega Drain -- therefore, it's important to evolve Gloom prior to Level 23 if choosing Bellossom.
It's a good idea to replace Mega Drain with Grass Knot as soon as you defeat Ramos, as it's almost always a better move. It's up to you if you want Grass Knot instead of Magical Leaf on Bellossom, but it's not nearly as important as giving it to Vileplume. Once you reach Dendemille Town, you can use Heart Scales to reteach Leaf Storm to Bellossom, giving it a very powerful move to use -- most Trainers don't have more than a few Pokemon, so the only downside is low PP.
Once you find the TM for Energy Ball on Route 20, the rest of the Grass moves Vileplume / Bellossom learn are irrelevant. The level at which you evolve Gloom only matters until this point.
Tl;dr the best level to evolve Gloom into Vileplume is Level 23, because it's then it learns Mega Drain, the Grass move that will tide you over until Ramos. The best level to evolve Gloom into Bellossom is around Level 21 - 22, so it can learn Magical Leaf (which will potentially carry it until Dendemille Town).
