Yes, I could just fill in that I have the same HTTP 403 problem with
gSync 1.4.0. For me it started today when I tried to add a second
GMail account in gSync. It asked for the account name and password,
but followed with error.
The problem stopped once I removed the second account (by clicking the
minus sign). But when I went for gSync/Tools/Reset gSync it all went
_worse_! Now as even my first account was removed, I clicked the plus
sign to re-add it - Filled in the account name and got the #%"€%"#
HTTP 403 in the face.
I'd be more than happy for a quick turnaround fix to this as it's now
unfortunately unusable - It won't sync any calendar for me..