GPs are a unit acquired for specific accomplishments in game. They are not produced by buildings and cannot be purchased directly with Drachmas. Instead you must participate in Global Quests, Campaign Quests, Solo PvP Quest, and similar events to earn these. Once you have acquired these, they may be spent in the Market primarily to purchase Articles for more advanced unit types. Note that this is the only way to unlock these more advanced units and you are thus highly encouraged to participate in any event that provides GPs.