'emerge gpa' produced a somewhat-working GPA installation, but when signing or decrypting, it would not prompt for a passphrase. Instead, it would immediately say "Wrong passphrase!"

I solved this by adding the gtk flag to the pinentry program, adding this line to /etc/portage/package.use:

>=app-crypt/pinentry-0.9.4-r1 gtk

and then ran:

eselect pinentry set pinentry-gtk-2

It took me quite a while to figure this out. It would be nice if you could add some kind of clue to the end of the 'emerge gpa' output. Even just a note like "be sure pinentry supports your GUI environment" would help put the sysadmin on the right track.