I used gContactSync-0.1.3.xpi for several months without any problem (other than the bizarre icons in the toolbar). I use this to keep my laptop, my desktop, and my wife's laptop synchronized with our Google contact (we both use the same contact list on Google).
I recently installed gcontactsync-0.2.3-tb.xpi on these three computers. A couple of days later, all of our Google contacts had been deleted.
I restored them and resynchronized all of our machines. This morning, all of our Google contacts had been deleted.
I restored them and resynchronized all of our machines. While I was synchronizing my wife's laptop, it deleted all of our Google contacts.
The first two times I set up gcontactsync-0.2.3-tb.xpi, I set it to synchronize all of our contacts. This morning, I started from zero, and set it to synchronize "My Contacts" only. This did not appear to make any difference. gContactSync still deleted all of our Google contacts.