The term "Guild Gun" has nothing to do with the American Custom Gunmakers Guild although the organization now has a stamp available for marking guns made by ACGG members. Dphillips, you are basically correct about the ACGG, but I don't think that is the context referred to here.
Atkinson is close to right with his explaination. The theory is that an apprentice made up the gun to pass his "master" status. In my experience, the term "Guild Gun" is used to describe any no-name German, Austrian, Belgian or Swiss shotgun or rifle that exhibits some degree of quality in manufacture, but nobody has ever heard of the maker.
I currently have a 16 ga. Austrian boxlock sxs by a M. Ogriss and a Belgian proofed French 12 ga. sxs marked LeCotere a Lyon. Both are lovely shotguns in quality of fit, finish, construction, stock design and engraving. I know of one other gun by Ogris, a single shot rifle.
I built a custom 16 ga. A&D type boxlock on a German proofed no-name gun a few years ago that I would swear was J.P. Sauer components. It turned out being a real dandy.

As for Cape guns, I never handled a sxs that felt right to me, just out of balance. I do have an O/U 16x8mm backaction sidelock hammer gun by Peterlongo, Insbruck that is a delight to handle and a marvel of detailed craftsmanship. Folding leaf barrel sights, a pop-up tang peep, sinlge-set trigger, rebounding locks, and lovely floral engraving. Unfortunately I have not been able to determine what the 8mm rifle barrel is chambered for. I've done a chamber cast and tried various cartridges in the chamber to no avail. It is a very fine combo gun and maybe one of these days I'll get it figured out.
If one can recognize quality and knows how to evaluate a continetal gun for wear and determine chambering, there are some really good buys on these so called Guild Guns.
Personally, when I go deer hunting I'm not going to shoot birds and when bird hunting I'm not interested in deer. Although last Weds. hunting antelope I flushed three coveys of Huns closing the last ranch gate of the day and sure wish I had a shotgun along.