The Adamastor you fought is an Archvillain and is available to everyone. Look under Objectives > Fight a Boss > Banished Pantheon (40-54).
GM Adamastor is not available. The only GMs available are the Kraken, the Ghost of Scrapyard, and two of the four DE monsters if you buy the DE monster unlock pack.
I did a mission yesterday that had Adamastor in it as an EB.
How do I get him, and what other GMs do we have available?
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