Hello,
Were you trying to rename a group that you created, or one of Google's system groups (groups that it made for you)?
gContactSync allows you to rename any groups that Google does, which means you cannot rename the system groups: "My Contacts", "Family", "Friends", or "Coworkers". I specifically wrote the backend to break before trying to rename or delete the group, but forgot to prevent it from getting that far.
I can detect system groups, so I could ignore the change or change it back to its original name. If I ignore it then I fear there will be bugs filed saying renaming groups doesn't work, but if I change the name back that will also result in bug reports.
I am reluctant to put any new dialogs or preferences explaining that you cannot change system groups because gContactSync 0.2 was already translated into one language and it looks like a few other localizations may be in progress. I could at least add a hidden preference and put it in the GUI for version 0.3.
I filed Bug 21002 for the technical details and my progress.
EDIT: If you click on the group in Gmail it should explain the purpose of the group in your native language.
Thanks,
Josh