GPP is short for Gross Panopticon Product. GPP plays a factor in determining the rank of your Panopticon in The League of Panopticons.
In order to contribute GPP to your Panopticon you must donate resources, which can be done in two ways: donating resources after completing a mission, or donating resources via Window on Liberty in your cell by selecting "Donate for the Greater Good".
The GPP value of each resource is determined by the type of resource and the rarity of it. The higher the rarity and value of the resource, the greater the GPP value of it.