\x00 chars are not allowed in C (end of string) so in GTK. But we can get from gpg encrypted messages.

src/common/connection_handlers.py

@@ -1606,6 +1606,8 @@ class ConnectionHandlers(ConnectionVcard, ConnectionBytestream, ConnectionDisco,
 			keyID = gajim.config.get_per('accounts', self.name, 'keyid')
 			if keyID:
 				decmsg = self.gpg.decrypt(encmsg, keyID)
+				# \x00 chars are not allowed in C (so in GTK)
+				decmsg = decmsg.replace('\x00', '')
 		if decmsg:
 			msgtxt = decmsg
 			encrypted = True