This has to be one of the most-asked questions on the touring forum! :-)

I've ridden both the GAP and C&O on 26x1.25 tires, and indeed was just on a section of it last weekend on those same tires for a day ride. I'd recommend wider, though. The GAP is a good surface of crushed stone, but if it it gets wet it can be a bit soft, especially if you are carrying a load. The C&O can have tree roots, rocks, and lots of puddles if after a rain (like last Sunday, we were dodging puddles a lot). Wider is definitely better for both of these trails.

Oh, and be sure to ride the Western Maryland Rail Trail when you get to it, to bypass part of the C&O, which is about 20 miles of glorious asphalt, split on both sides of Hancock.