You selected 20 outreach emails. Sending more than 15 emails at once from one Gmail can push your emails into spam or get the account flagged by Google.
For best results, keep it at 15 or less per Gmail.
Sign in with the Gmail you want to send outreach from. Guest Poster only asks to send emails for you. It can't read your inbox.
If Google shows "Google hasn't verified this app", click Advanced and then Go to Guest Poster. When Google asks, keep the Send email on your behalf box ticked.
Outreach is sent from the connected Gmail. Every Gmail logs into the same sheet.