Pumpkaboo in Pokémon Go evolves into Gourgeist. There are four different sizes for Pumpkaboo, but each has the same evolution requirements as the others. There are two ways to evolve your favorite Pumkaboo into a Gourgeist, but one of the methods is a questionable choice that you need to be careful about choosing.
The standard method of evolving a Pumpkaboo into a Gourgeist occurs when you want to give them 200 Pumpkaboo candies. You will have to earn this candy by catching and transferring Pumkaboo, completing missions to earn Pumkaboo candy or using rare candy on it. However, all of these methods take time.
The alternative method is to trade a Pumkaboo with another trainer. When you trade it, the person who receives the Pumkaboo will evolve, becoming a Gourgeist. So if you want a Gourgeist, you need to recruit a friend to catch one for you and transfer it to you. However, the downside is that when you trade a Pokémon with another trainer, the Pokémon's stats change. There is a greater chance that Pokémon with better stats will get worse when the trainer receives it.
Among the methods, you'll want to trade with another trainer if you both have a low-stat Pumkaboo. If you catch a Pumkaboo with perfect IVs, it's best to wait until you get all 200 Pumkaboo candies to evolve it yourself.