The way I understand it, if you level a pet to 50 before it has any children then its GP will increase. Well I leveled with this Puma: http://beastkeeper.com/pet/147819

It was GP1 and now it's still GP1. She was born male, I don't know if that's an issue.

Thanks.

Pet's GP does not change, it's an inborn characteristic that can't be altered. However, lvl 50 pet can give birth to a baby with +1 GP grade if all of the conditions are met.

Please check FAQ for the rules of GP inheritance and acquiring.