This complaint isn't really against GMC itself but rather the consequences it could have if it isn't monitored correctly, your points being:
- Causing lag
- Bypassing tag and gifting/selling items
- Taking the resources from other dimensions
First of all you should take into account that only a minority of the GMC players have been involved in such cases so far. Wouldn't it be overboard to "punish" all the players that chose to support the server for something only a few people have done?
Then, the first and third point aren't only caused by GMC, as bruny pointed out, lag is more often than not caused by regular players and from a new player stand point, there's no difference between a dimension cleared by a few gmc players than a dimension cleared by players that have been playing longer than you have and got there before. Servers are asynchronous
Since lag is a major issue on any minecraft server, we are always working on finding the sources with the help of profilers and instructing the players about best practices when they make a laggy setup by mistake. Whether they have GMC or not.
About the tags issue, it's against the rules and punished heavily. Our staff keeps monitoring the GMC players to be sure that it doesn't happen and in case of doubt the GMC player gets restricted access to the market, so besides specific cases it shouldn't impact your experience of the modpack. We've been doing a good job with that on all our other modded servers so it should be good here as well.
About the dimensions, we have a mining dimension and that one and the other dimensions get wiped every now and then, so a dimension should never be totally empty. And if it happens to be you can make a post here and we will reset it.
As you can see, we are already aware of the issues you brought, since we have been handling with GMC players on all our other servers for a long time and so there are measures in place to prevent their behavior to impact your experience of the modpack.
Also, if a vote poll is opened, think this thoroughly and from all the perspectives, since as a non-GMC user voting against is the easy choice and could cause a bias, we should aim for a solution that satisfies everyone.