Google's GMail service requires some extra configuration to work with Pegasus Mail. In particular, GMail has chosen to champion an authentication mechanism called OAUTH2, which is complex and not widely supported at the time of writing. To GMail, any application that does not use OAUTH2 is a "less secure app", a position that is both insulting and controversial. We are attempting to determine whether we can support OAUTH2 in future, but until that time, to send mail via GMail using Pegasus Mail, you'll need to follow these steps:
1: Login to the gmail web interface and click the icon at the extreme top right-hand corner of the screen - on my system it's a stylized stick figure in a blue circle, but it can be changed, I think. Select "my account".
2: In the "Sign-in and security" group (the left-most on my system) of the window that opens, click "Connected apps and sites".
3: Locate the slider called "Allow less secure apps" and set it to "On". Disregard the warnings Google will raise about this - your connection will be SSL-secured anyway, so the risk of interception is small.
4: In Pegasus Mail, tell the program to connect to "smtp.gmail.com", port 465.
5: In the SMTP "security" page, select "direct-connect SSL", and enter your full gmail e-mail address (including the '@gmail.com' part) and password in the second option in the authentication group (the option entitled "Login to the SMTP server using the following details").
6: Pay special attention to the case of the e-mail address and username - BOTH of these are case-sensitive in this situation. In essence, whatever GMail displays when you login to it is the case it expects you to use during authentication.
That should be all that's necessary to handle sending. Enabling POP and IMAP is done separately, and more easily from within GMail itself.
-- David --