There is no guarantee that HTC will do a Gingerbread update for the Desire at all. There is the effort required to port HTC Sense over, and then a few weeks of testing (you would not want a half baked OTA update, right?) and finally the carrier approval process which would take another few weeks. All this is required of a manufacturer and carriers who are more interested in getting you to buy a new phone in a few months' time.
In short, I am hoping to see an official Gingerbread update for my Desire, but I am not really expecting one.