For those walls, see if you can't find a wall stamp without doorframes. You want something that will tile without looking like it's tiled, and the walls you have now don't do that. They look like movable partitions to me.
I'm at work at the moment, so I can't really whip up a useful example, unfortunately.
Give some thought to frazier's suggestion of making the entire map in GIMP and forgetting about trying to come up with decent tiles.
I'm not sure how well it translates to the GIMP, but try your hand at using this tutorial for making dungeon maps in Photoshop:
(Photoshop) Create a dungeon floorplan in minutes (dial-up killer)
And check out http://www.cgtextures.org for some really nice photographed textures. They don't tile, but they're enormous, so it doesn't really matter--you can usually just crop them to the size you need, or if you're exceptionally clever you can hide seams underneath walls.
I found the combination of those two resources, along with a few pieces of art from http://www.dundjinni.com very useful in creating my first dungeon battlemat for a mapping challenge a couple of months ago:
March Entry: Wrong Turn Cellar
Bryan Ray, visual effects artist