Good news: As of Version 4, WP Issues CRM supports fully secure connection to Gmail using OAUTH2.
Please see this link for a full explanation of how to get the most secure possible connection to GMail using OAUTH2.
Setting up OAUTH2 is intrinsically complicated. It requires you to open a developer account at Google. This may be worth the trouble in a multi-user installation of WP Issues CRM. Alternatively, you can easily connect WP Issues CRM to Google using a legacy IMAP connection. This older superseded post explains how to use WP Issues CRM with Google and a legacy IMAP connection. Please note that the superseded post was written before we upgraded WP Issues CRM to support OAUTH2.
Bottom line: If you want to use Gmail with WP Issues CRM, you have two options: (1) OAUTH2 (more work and more secure); (2) Legacy IMAP (easy and a little less secure — Gmail will call your app “less secure”).