In general, GAM access is not required in order to use HTL BID, meaning Hashtag Labs does not need access to your GAM.
Features that do require GAM API access include:
Line Item Generator
HTL BID can automatically generate Prebid Line Items. This requires API access to create the new Orders, Line Items and Creatives.
A/B Tests
HTL BID can automatically run A/B tests. This requires API access to to create key-values for each “build”.