General number field sieve is slower then exhaustive search for 'small' numbers?
First of all, the number 120 that you got is completely meaningless. There is a scaling factor that can only be determined experimentally. Even worse, there are lower order factors which are probably very significant for such small numbers. Finally, the running time you quote is heuristic – it's just an educated guess.

You are correct that GNFS is a lot less efficient than trivial methods for small numbers. That's why you don't use it for small numbers. The better asymptotics kick in only for large enough numbers.