After intensive testing I came to the conclusion that surfaces with textures defined in GDL directly are not shown in sections/elevations where you've set them to show Surface Textures. (showing textures from normal materials work fine, tho)
This somehow matches the experience of James.
It is worrysome that GS has not learned from the mistakes of the past (BMATs anyone?)...
Will this functionality be delivered, GS? @Peter Baksa ? (: