GB I thought was already integrated into Pakistan? That limited autonomy bill is a complete sham. It won't really, as the ground reality stand both forces are staring each other down, Both can claim whatever they want to claim in their own constitution. If there is an invasion and one side can occupy the other, it's not going to make any considerations to paperwork.
What you thought would be an Indian reaction? Opposition will create a whole scene and seek action from GoI considering our stance on PoK.
IMO, India would be in a bigger fix as we have legal instrument of accession, Pak stand is just moral (sham).