If you are arriving at Gardermoen from outside Norway, you used to be forced out through passport control and luggage reclaim. If that’s the still the case, you could head downtown on the airport express (Flytoget) for £18 each way. So about the price of a half pint of Ringnes at the airport 😉
Airport is really dull (albeit architecturally interesting), so well worth doing something.
A trip up to Holmenkollen (Ski jump overlooking the Oslo Fjord and city centre) is probably the best thing to blow away the cobwebs and do-able in about an hour and a quarter from the airport if public transport goes your way Ski Jump Link.
Alternatively take a wander down Aker Brygge and the waterfront Waterfront link. That’s about 45mins from the airport.
Oslo is nice enough, and rarely somewhere people go as a dedicated destination, so I’d make the most of the opportunity to see if you’d consider going back.
PS Cabs are horrendously expensive and slower than the train to downtown, so really don’t bother.