I agree, there is nothing to lose, there is also nothing to gain. The way I see it the only people that this document means anything too is the GPA. It looks to me like a glorified NAP, a NAP that is unneeded since GPA states that they will not have any non defensive military action against anybody. That very statement in essence means that the GPA already has a 1 sided NAP with all of Planet bob.
Your right that GPA has the right to play the game its own way, and they are free to do that, and signing or not signing this document will not effect that right in any way.