There are several things I would suggest for your team, but most of this is just personal opinion, so you need not take the advice if you don't like it!
Consider switching out your mantine's held item for a damp rock, especially if you are competing under conditions in which repeat held items are not allowed.
If you definitely want your team to be rain dance-based, try using Politoed as your lead in place of Mantine. It may be OU, but there's a reason for it. Politoed has high Sp. Defense and decent Sp. Attack. If you're looking for a rain dance staller, Politoed is definitely a big help and you don't have to waste a turn using rain dance.
I am suggesting the rain dance based team due to your Mega Manectric's thunder, it is a tricky risk even when mega evolved. I myself am not a big fan of lower accuracy moves but the risks are sometimes worth it because of the payoffs. Rain dance would nerf this risk altogether, and it would also give your gyarados a boost.
Instead of a Gourgeist, consider what I use:
Whimsicott@Big Root
Ability: Prankster
Nature: Bold
Max Attack, Max HP, 4 Defense
Leech Seed - Part of the sub seeder combo you were going for on your gourgeist
Substitute - sub/seeder
Giga Drain - Gets added bonus with big root + STAB
Toxic/Endeavor/Cotton Guard/Taunt
Toxic to wear the enemy down with an added status effect, Endeavor if your subs fail and you're in the red, Cotton guard to raise your high defense even higher, taunt to prevent other pokemon from setting up on you.
I like Whimsicott for his prankster ability, mainly. Even if the opponent has a priority move Whimsicott will go first, meaning they won't be able to take you out with one hit thanks to substitute. Send this guy out against physical attackers and he'll definitely be a huge annoyance. He's also great against walls!
