Hi Luis,
I would recommend using the Rest Management API (RestMan service) for achieving this. It can be easily published from the Gateway and can be a simple way to migrate.
Once published, the documentation can be accessed at https://<gatewayHostname>:<port>/restman/1.0/doc/home.html.
Note: this page is key to turning the exported bundle into an acceptable xml to be imported again
(9.1) Restman MigrateIN - ClassCastException | Layer7 API Management (broadcom.com)Hope that helps but if you still want to use the GMU, I will leave that for someone more experienced in that area.
Kind regards