Almost all uses of AES in GCM mode forego the use of a separate hash algorithm, essentially message authentication is built-in. Therefore providing a hash algorithm when it is used is nonsensical; whether Mikrotik should accept auth=null (or automatically assume it if GCM is used) is an interface question and would at least merit documentation. This might be a legitimate bug, even if only in terms of documentation.
Phase 1 and phase 2 are different. In ikev1 they are very different in terms of their cryptography; in ikev2 they are much more similar. Ikev1 generally doesn’t therefore allow the usage of GCM - at least not in the way it’s usually used. This a fairly good answer:
https://crypto.stackexchange.com/questions/74411/why-cant-aes-gcm-be-used-in-ikev1-phase-1
For ikev2, GCM mode may be used. There are quite a few vendors who don’t (yet?) implement this.
There is no marked benefit of GCM mode for phase1 over CBC mode with at least sha256 hashing.