Usually speaking, yes it is.
Considering that Gogoat and Gourgeist are both weak to Flying, Fire and Ice, something like a Talonflame would trash both of them in one Brave Bird. Also, Garchomp is doubly weak to Ice, meaning you'd have 3 Ice weaknesses on the team. Greninja is the only one resistant and it's not good for taking hits
The general point of a team is to for the members to balance and account for each other's weaknesses. There are teams that don't do that, like mono teams which only use one type, but that's different as it's setting a challenge. You could just make your team hyper offensive, but neither Gourgeist nor Gogoat are out right attackers so that wouldn't work either if you wanted either of them on your team.
For your purposes, it's a really bad idea to have both Gogoat and Gourgeist on the same team, so ditch Gogoat and go for something else that doesn't share Gourgeist's weaknesses.
