my old Toyotas had the fuel filler cap release on the floor under the drivers right leg. i think it works on a simple catch type of mechanism like the bonnets on most cars where you pull it and a cable is attached to this release catch thing and springs back and to close it you just press it back. sort of.
last winter on my old car i had problems releasing the fuel filler - embarrassingly i had actually pulled up to the petrol pump and then yanked the little lever as hard as i could - it moved a bit but was quite evidently stuck. it was the first time anything like this had happened to me. thankfully i had enough petrol to go home so i just drove back home to spend more time on trying to 'fix' it.
somebody on Facebook had jokingly said i should pee on the fuel cap. i had the idea of getting a bottle of warm water and just squeezing it all over the fuel cap and eventually it loosened enough for the cap to release. i couldn't be bothered going back into town so the following day i rechecked and it was still loose enough to open.
i don't really have much other experience other than that. but if i were you it may be possible running the car a bit the heat of the engine might loosen up the catches. otherwise perhaps try parking in a warmish car park for a bit, maybe find an indoor car park somewhere?! otherwise i dunno whether pouring warm water all over the bonnet may help? i'm guessing would have to be a lot of water though.... and then when it's all over the floor it's just going to freeze so might cause a hazard.
hope u can get it sorted.
sort of unrelated but i've found lately in the cold weather opening the doors is harder, the rubber seals seem to get hard and as i'm yanking the door open i can hear the seals just crack with the sound of the ice snapping - i hope the seals don't leak when it's raining

first winter with the GTI....... i shouldn't be worried, but somehow i feel a lot more care for the GTI than my other cars.....

am I mental or something?