Hello Samjon,
Some components such as OTK/API Portal Configuration, Passwords, Private Keys, Certificates that all exist on Gateway and accessed via Policy Manager can be moved with GMU, given these items are used as dependencies in services (Portal configuration meaning portalman service/policy fragements for plans/etc).
Do note that API Portal assertions, audits, logs, custom assertions, and un-referenced private keys/certificates in services, all will need to manually moved over to target boxes.