I am sorry if this sounds dense: But if you are sending the emails from server side, how are you getting to the user credentials / gmail login 2 step verifications etc.?
I am assuming you mean you send from the client UI but the extension talks to your server for scheduling the future sends etc. in which case you shouldn't have that much network bandwidth.
The user logs into his Gmail account on his own and uses the GMass extension, which operates within the Gmail UI, to create/schedule campaigns. On the back-end, GMass connects to the user's account over standard Google OAuth 2 to then send the emails.
