ET airport upgrade
 

I know people that have done it many times. However I am pretty sure they did it at the check in counter and not at the gate.
In many cases I believe it was more expensive than purchasing the business class ticket in the first place. But all depends on the route.

It's been quite a while ago, but I was told by the gate staff that they can't do it. It has to be the check-in counter as Goaguy mentioned. I imagine the transit counter would have the same power but they are often quite busy. This was all before they started to participate in the online bidding for upgrades, so they may not offer it at the airport anymore to force people onto the online system.

They offer it. Never seen a good value relative to the online upgrade (whose value can fluctuate wildly also).

Gate upgrades are always more expensive than the online upgrade offer. (they reprice the ticket to business class, not "upgrade")
If you really want to upgrade, do the online offer..latest 48hrs before flight

The minimum upgrade price is 450USD for the 4 hour flight.
I wonder if the are able to fill many of the 24 Cloud 9 seats on the 787-8

ET will fly out with empty seats if your upgrade bid is rejected. They are not an american based airline that fills the cabin with upgrades just for the sake of it!
If you really want an upgrade, put in the minimum bid. Most likely will be accepted (on this route, not much competition for bids)