Taken from this AskWoody article: https://www.askwoody.com/2022/i-have...nect-to-gmail/

First, turn “2-Step Verification” on in your gmail account and make sure it works. For me, this required at least two automated phone calls from Google to verify my identity. After that, I could access my gmail account with the gmail app with no problems.

Then select the “Security” tab on your Google Account, then look for “Signing in to Google”, and select “App Passwords”. Google asks for type of computer (Windows PC for me) and program (“Outlook” is NOT an option and “Mail” does not work; instead Select “Other” and then type “Modest”) Google then creates and displays a 16 character alphabetic password. Write down this password.

Then head back to Modest and enter the 16-character password you just got in both SMTP and IMAP/POP3's password boxes. The result should be a working Modest.