We even went so far as to create a "sister" guild for our GIMP characters. Participation in the "sister" guild is sanctioned and supported by our primary guild, and that generates continued interest. Having all of our GIMP characters in a guild makes it easy to tell if anyone else is around who wants to party with you. We also created a separate chat channel, and all of our regular characters joined the chat channel, so if someone wants to start up some GIMP action outside of the normal night, we have a way of organizing.
So, in order to make the "no grouping with non-GIMPs" rule work, you'll need to be organized. I can say that, without some level of organization, you are pretty much guaranteed of never finding another GIMP. At least not an intentional GIMP who plays by the same rules as you
Alternatively, you might try dismissing the "no grouping with non-GIMPs" rule. This would allow you to personally see how far and how effectively you can play a GIMP character. I'm not sure that it would be as much fun, as there would be no one to share your exploits (and faceplants) with. You also might get some "interesting" critiques of your character from people who don't understand what you're doing. However, it is an option for those who want to try out a GIMP character but don't want to organize a group to do it with.
These rules aren't set in stone, they are simply the guidelines we used. Feel free to add to, subtract from, or modify them to suit your own preferences.