apt install libpurple-dev libgpgme11-dev
autoreconf -i
./configure
make
mkdir -p ~/.purple/plugins
cp src/.libs/pidgin_gpg.so ~/.purple/plugins/
Select Tools > Plugins, and enable the GPG/OpenGPG plugin. Select configure and choose your GPG key.
gpg-agent needs to be enabled for this plugin to work properly. You may need to restart pidgin to be prompted for the key passphrase after enabling this plugin.
This is not the official branch. I'm maintianing this fork simply for maintenence (the original branch did no build with recent version of autoreconf). I'm am not actively developing pidgin-gpg, maintly making sure it keeps working and building. Releases > 0.9 should not be confused with those of the original author (though the original author seems to have stopped all development).