Why does they 'Authentication' hash drop down disappear if I choose any of the GCM encryption types on the Phase 1 Transform screen?
*** compared with ***
Authentication and encryption occur simultaneously.
When you select the one, it already knows the other.
The AES-GCM cipher suite(s) include authentication as a part of them, so selecting an additional authentication cipher is not necessary.
Or rather, they specify one in and of themselves. If you could select a different authentication cipher, it would break the protocol, and would not work.
WatchGuard Customer Support