I MAY have actually figured this out, as it has been making me nuts for the past few days since I made the switch from Quickbooks Desktop to Quickbooks Online.  I actually went and used my business email address to create a gmail account. I was able to use my exact address even though it is not "@gmail.com" After doing so I was able to add that one into the "add gmail account" in the from dropdown when sending and invoice.  For the past few tries, it seems like it is working!