I’ve always felt the best bar would be a tangible cost for registration, not player minimums.
Every time minimums are brought up, a lot of people complain their guild is just a bunch of friends or family members and they think it’d be dumb if they were excluded from a game mode due to that. They have a point. The rewards for “just being in a bracket at all” aren’t much worse than the rewards for “winning your bracket” for 99% of players.
So I think GW as we know it is a good mode for some game that’s not Gems of War. It only works as-is if every participating guild is competitive. It’d be better if a World Event ran in conjunction with GW. I think if they did that and removed rewards for being low in your bracket, it’d discourage most casual guilds from bothering. More so if a cost was attached to registration, say a 5m gold entry fee per guild. That’s not a lot at all for a competitive 30-member guild, but a 6-person family guild won’t see the benefit. But they wouldn’t feel like they were “losing rewards” if there was a concurrent event running and GW was known as “only worth it if you’re competitive”.