Gloom

Razor Leaf + Sludge Bomb and Petal Blizzard or Moonblast

Razor Leaf is very powerful with STAB, and should be taken advantage of whenever possible. Acid doesn't provide a large difference in energy generation and deals way less damage, so it's not as preferable. 

The poor energy generation from Gloom's fast moves results in infrequent use of its rather expensive charge moves. As a result, Sludge Bomb is used a majority of the time simply because Gloom would likely faint before getting enough energy for Petal Blizzard or Moonblast. In the rare cases where Gloom would endure a prolonged fight, access to two Sludge Bombs would pressure shields more than just using one expensive charge move. 

Great League: 2 / 5

Gloom is one of the many Grass-type contenders which makes it difficult for it to stand out. While access to the powerful Razor Leaf is a plus, a lack of a cheap charge move energy-wise makes Gloom rather slow when it comes to shield pressure and overall damage output. Other Grass-types with the Razor Leaf/Leaf Blade moveset tend to be more favored such as Bellossom, Victreebel, and Tropius. A max CP of 1681 also makes Gloom somewhat of an expensive choice as it may need to be powered up significantly depending on IVs. Overall a decent pick for Great League, but probably not the first due to many better options. 

Ultra League: 0 / 5

Gloom doesn't have the stats for Ultra League. 

Master League: 0 / 5

Likewise.