If you play you earn 1 credit that is applied to the character that played through the scenario.
If you GM a scenario, you earn 1 credit that can be applied to any character that hasn't played through the scenario.
You receive GM or player credit regardless of the order you play/GM the scenario. You may not earn more than 1 player credit and 1 GM credit regardless of how many times you GM or play the scenario. You are free to use PPP to seat legal tables, but if you already have earned your credits you do not earn any additional ones.
Thanks!
Hyrum.