I just Googled it, and it looks like I pretty much made up the term. ._.
Dummy GTS servers basically began life when someone figured out how to emulate the protocols and stuff the real GTS uses, meaning they could run their own, limited GTS server to do a bevy of things. People mostly used this as a means to distribute clones of events or outright hacks, but some enterprising individuals used this as a way to check the legality of their Pokemon, or better yet, semi-legally file trade their Pokemon without the use of a nasty external device. You'll find the majority of those people at PokeCheck, which is the place I'd recommend going if you're interested. Pokemon has spoken out on dummy GTS servers, and, at least to an extent, and they claim they're cheating, so you might want to be wary. Make sure anything you download is legit, have the utmost care if you're ever entering VGCs, and don't directly link to files in Wi-FI Trading Center; KA didn't want that happening, last I checked.