In the bug report of Debian ( https://bugs.debian.org/869416 ), I found that gpg is invoked only with LOCALE variable and no --display or --ttyname.
The manual gpg.1 only addresses HOME, GNUPGHOME, GPG_AGENT_INFO and PINENTRY_USER_DATA.
Actually, gpg supports --display, --ttyname, --ttytype, --xauthority for the session env vars, but those are not explained in docs.

It's good if we can document how to invoke gpg. In the particular bug case, gnome gcr library invokes gpg --import.

(I know that we should use gpgme library, in general.)