First confirm you don't have existing old certs which may conflict.
1. Open Device Info app, select Cert Mgr from Pref menu. Look for the three certs Grabber mentioned and delete them by selecting each and scrolling down to delete. This should clear out old pem files and their associated symlinks which are obsolete.
Then install the new pem files.
1. The new pem files in the zip - unzip onto your PC
2. Connect device to pc add USB, copy 3 pem files to device (pc lists as E: drive usually)
3. Eject e: drive to safely disconnect device from pc
4. Open phone's Device Info app, select Cert Mgr from its Preference menu.
5. Touch symbol at bottom with plus sign to add a cert. YOU SHOULD SEE A LIST OF THE THREE GOOGLE PEMS YOU ADDED. accept and trust each one.
At this point, You should be able to add the IMAP account and sync mail.
1. Use Internalz Pro app (installed on device) to navigate to /var/ssl/certs and /var/ssl/certs/trustedcerts to delete the old files manually. I've explained how to do this in another post, refer to it as needed.
Good luck. It is doable.