Am I correct in saying that the pedal doesn't have flats for a spanner, and also that the rounded bit is the tool, and the hex interface in the pedal is still good?
I don't rate GT85 as a penetrating fluid, I find Plus Gas much more effective. Personally, if possible, I would put the crank arm in a vice, and use a good quality hex key with a tube over the end for leverage. That way, you can put one hand on the top of the hex key to try to avoid it jumping out. It's possible to do the same with the floor and a wall, but you run the risk of punching the wall when the hex key moves. Also, you have to be sure that you are turning it the correct way - pedals have different threads depending on which side of the bike they are on.