The gtk-sharp-module eclass should be updated: starting with gtk-sharp-2.12.21, the SRC_URI ends in "gz" instead of "bz2". To do this, check if the gtk-sharp version is >=2.12.21, and if so adjust the SRC_URI accordingly: SRC_URI="${SRC_URI/%bz2/gz}" Thanks!
On further looking, perhaps I missed the boat on this one — it looks like this eclass is on its way out. (The issue is still a thing, just not particularly important because of that.) (I could be wrong though.)
Eclass removed.