Why? Each of those tokens is O(1) and it is predicting each one incrementally, taking into account the ones it has just generated. So the full answer has taken O(m) where m is the number of tokens.

If it is possible for GPT to do 1+1, it can do a large number of them incrementally. It's not smart enough to do it all the time (you'll have more success if you encourage GPT to have a train of thought reasoning) but it's possible.