Misplaced texture problem

I don’t know how it happened, but there’s a texture that’s been mixed up with another in my GMod.

As an example, a part of the Goldrush map normally looks like this:

Those gray stone walls have somehow been replaced with a texture of building windows. It’s like this with many other maps; I keep seeing windows where this normal texture should be. Not sure how that happened…

Any ideas on how to fix it?