I am about to have my friends adventure in a "generic" dungeon, using GURPS Dungeon Fantasy supplements. I know but a little about Banestorm setting, the "default" fantasy setting for GURPS. In case the dungeon adventure should last some time and expand its scope, shall I be able to use Banestorm as my general setting? Does Dungeon Fantasy introduce concepts, magic variations and the like at odds with some important aspect of Banestorm? (For instance, DF9: Summoners modifies slightly the general magic rules to make summoners more "usable".)
I think that there is enough empty space on Yrth (Banestorms world) to place a good old dungeon nearly anywhere. Maybe deep in the orklands or in the north? I would make the decision according to the background of your party members. And of course of the ongoing adventure.
When incorporating a slighlty different magic system I would give the character an appropriate background. Maybe he/she is from a foreign country, belongs to a new and different magic shool or is even from another world. I once had a "free wizzard" in a campaign on Yrth who came from a country far far in the east. He didn't used the spell oriented magic system but had general levels in some aspects of magic. This worked very well and the local magic guilds where very interessed in him. :)
I can see no hindrance introducing any of the Dungeon Fantasy templates into Yrth. With a decent background to the charakter, this can be done easily.
GURPS is designed to be system-compatible with everything. The whole point of Banestorm (as much there is one) that stuff from other universes gets transposed hither and yon.
Conceptually, you can either have a standard Dungeony dungeon with heavy wood doors banded with solid iron, or you could embrace the Banestorm setting completely, and have your adventurers round a solid stone corridor's corner to face five very confused storm troopers and a short beeping robot.