What bargain has gawain agreed to?

The green knight made the bargain with Sir Gwain:

I will suffer any blow you care to give me if in return you allow me the same.

So Gwain cut off his head.

And the green knights body picked up its head and put it back on its shoulders.

And as the green knight was going to strike off Gwain's head Gwain started to bargain rather hurriedly. He got the green knight to grant him a year of life and a three part riddle to solve to save it completely.