As of now none of the libgdiplus ebuilds in the tree support modular X deps.
Created attachment 71815 [details, diff] libgdiplus-1.1.9.2-modular-fix.diff Adds modular X dependancies as well as other dependancies from the configure.in file and seperates RDEPEND and DEPEND
Fixed in 1.1.9.2 and the just added 1.1.10. Marking FIXED, thanks for the patch.