I played a bit around with the textures used in Alien Swarm. I changed the headers of their VTF Version 7.5 to a compatible 7.2 (the one GMod is mainly using) but it failed. Seems like it's not the header but the data attached which causes GMod not to load the texture.
(Btw: I used photoshop and Nemesis VTFLib to load the modified vtf and it did fail. If VTFLib can't handle it, GMod can't too).