To unlock a german cupola mg from a Relic tank by changing RGD files, you need to do this stuff:
1) You need to assign a machine gun weapon to the tank, either from the ebps or a slot item given per upgrade, like Relic does.
2) You need to enable the crew member, look at the crew_ext property of a Relic tank with gunner, like the Panther or Pz. IV. Relic usually enables it when purchasing a certain upgrade or vet level.
Supposing the model already has the MG ready to be used, if you want to make them appear by default, make sure the aforementioned crew element have "sync_on_create" as "true" and make sure you add the mg from the slot weapon that loads to an unused hardpoint or just copy the add slot function from the upgrade on the action_apply_ext->actions->upgrade_actions or squad_action_apply_ext->actions->upgrade_actions.
Maybe this is too much XD...