We currently run GCP builds in parallel to our normal ones, e.g:
https://treeherder.mozilla.org/jobs?repo=mozilla-central&group_state=expanded&searchStr=B-gcp

With the GCP migration now under full swing, we're going to want to turn these into the regular builds and disable the old ones.

We'll want to test release promotion extensively, as well as make sure the new builds don't introduce problems in our tests.

We'll also want to switch all the other build variants over to GCP at some point (asan/tsan, ccov, etc).. Though I'm planning to file a follow-up for those.