even if they force you to check it at the gate (which is unlikely) i've never seen anyone have to pay the fee at that point - just get in line early so you can get the overhead locker space when boarding.
what happened to me in Australia (I believe it was Virgin Blue), They forced me to check my carry-on at the gate but didnt collect any fees. They issued me a luggage receipt and upon landing in Cairns, we were directed to go to baggage claims where we had to fork over what was about $30 USD for the single piece of luggage.
I mean it makes sense to collect the money afterwards or else you slow down the boarding process.