in the GDK there are 3 "clipboards"
to paste contents of PRIMARY, user need to do middle-click
to paste contents of CLIPBOARD, user need to do Ctrl-V or Shift-Insert or right-click->Paste or menu Edit ->Paste
i think that at the present the most used/known/common way is the second
but on "Copy path", the Geeqie uses PRIMARY and, imho, this seems unexpected that i have copied, but can not paste it in a "common way"
so i suggest
sed -i -e's/GDK_SELECTION_PRIMARY/GDK_SELECTION_CLIPBOARD/' ./src/utilops.c