Thank you very much Davegrow.
After official Samsung Kies update to GB, my mobile was working fine. I played with some system fonts and my mobile has gone into rebooting loop. I rooted, my phone and used Link2SD to increase my Internal memory. I revived my phone successfully, but I always got this "Gtalk authentication failed" error and couldn't download any apps from Market. I was thinking to downgrade my mobile to Froyo. Great thanks to you Davegrow. You saved me much mental agony.
Thank you very much.
I am loving my Galaxy FIt, its a great phone.