GP is the currency on GURPG. GP is used to purchase items and weapons from the shopping district.
GP can be obtained in two ways, the most common of which is clearing dungeons. Just like how in the game, you will receive a rating based upon how well you completed the dungeon. You will receive one similar upon the clearing being approved. The higher you score, the more GP you get. The second method is competing in the arena. You gain GP by winning and receiving a good score in the arena.
More detail on specific GP earnings will be in the corresponding topic. (Dungeon Info and Arena Info)