There is already conditional a fetch restriction in place (with USE=gog). That doesn't prevent the file from being mirrored. It should be changed to this: RESTRICT="mirror bindist gog? ( fetch )"
+ 18 Apr 2014; Julian Ospald <hasufell@gentoo.org> duke3d-data-1.0-r1.ebuild: + add mirror/bindist restriction wrt #507952