They are essentially paying a tax on behalf of Visa/MC/Amex (none of which are HQ'd in SF). It's actual more than a "double tax" since more than 50% of the processing fee is split with them but not a literal "double tax" imposed by SF.
Gross receipts is all revenue not including any cost or expense https://www.investopedia.com/terms/g/gross-receipts.asp
As for adjusted revenue, do we actually know for certain that gross receipts applies to net revenue and not adjusted? Or is Dorsey just “hypothetically” spreading FUD?