There is a support web site for gContactSync. You should ask there.

I went to the support site. There is no provision for groups. I need to pull addresses by their groups. What alternatives to Thunderbird exist that can do this?

Thunderbird did group my Google address book by group. How can I download an earlier add-on that did that?

Ask in the gContactSync forum.

In desperation, I uninstalled the new version of Thunderbird and gContactSync, installed the recent version that I had on my old computer, copied and installed the Profile from the old computer (emails had continued to sync) It is working very well. In the process, I did not allow gContactSync to touch the addresses. This has been a long and painful process. But the Thunderbird I knew was so good that it has been worth the effort.