Hello @young and welcome to the RStudio community!
That is an interesting case! I have to admit that I am not familiar with using Gmail as an SMTP server. It is definitely unfortunate to have Connect classified as a "less secure app"
I suspect part of this is because Gmail is a unique beast. Are you using an "App Password"? It is not super clear what the solution to this problem is, but I suspect part of is that "more secure apps" have an OAuth or other API relationship with Gmail directly.
Our implementation is generic SMTP, so it may be that Google wants us to be more of an explicit "Gmail" provider.
This thread on StackOverflow suggests that maybe you just need to enable SMTP.SSL in the RStudio Connect configuration, though?