byzantium: Add gdm3

byzantium.yml

@@ -33,6 +33,8 @@ firefox-esr-mobile-config:
   url: https://source.puri.sm/Librem5/debs/firefox-esr-mobile-config.git
 flash-kernel: {}
 gcr: {}
+gdm3:
+  url: https://source.puri.sm/pureos/packages/gdm.git
 geary:
   url: https://source.puri.sm/Librem5/geary.git
 gedit: {}