Google are now forcing 2 Step Verification on a lot of users.
To view your Gmail within Pegasus, you will need to disable 2-Step Verification within Gmail's Security settings.

Once you have disabled 2-Step Verification you will then have the option to allow Less Secure Apps Access within Gmail's Security settings which is a requirement for using Pegasus with Gmail.

Future versions of Pegasus will hopefully fix this issue.
The sooner the better

NOTE: If you don't use Pegasus with your Gmail address for a fixed period, you will have to reset the Less Secure Apps Access option in Gmail. I don't know what this period is, but I reckon that it is about one month.

(I'm not sure if any of the above applies to IMAP)

Alternatively you can have Google create an application password that you then use in Pegasus Mail. This allows you to get the benefit of 2-step verification for web access while allowing a program like Pegasus Mail to bypass it. The app password approach requires that 2-step verification be enabled.

As for where the development of Pegasus Mail is on this read the most recent developer news article here: http://www.pmail.com/devnews.htm