Unfortunately, GPA will have to be removed from Whonix 18, as it has been removed from Debian Trixie due to lack of maintenance. See Debian Package Tracker, https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1089914. The suggested replacement, KDE’s Kleopatra, pulls in somewhere around 280 MB of dependencies due to reliance on KDE libraries.