GPP in Fantasy Football stands for 'Guaranteed Prize Pool.' Simply put, GPP is a type of daily fantasy sports tournament where the prize money for winning is guaranteed, regardless of if the tournament pool you enter fills up.
Guaranteed prize pools have a different pay structure than some of the other tournament styles. In a guaranteed prize pool, a small percentage of the participants actually win money. This is usually a round number like 10% or 20% of the pool entrants. The prizes are also adjusted to the position you finish in (i.e., first place wins more money than second place and so on).
There absolutely is strategy involved. It might not seem that way because you are choosing one lineup and hoping for the best. However, because you are given a set budget to work with, you need to strategically choose your players, to maximize the value you are getting from each player. Researching matchups of players vs. the defense they are facing is one way to find the best value pickups.
Oftentimes, on daily fantasy websites like DraftKings and FanDuel, you can see what a player's projected points are supposed to be for a game, and how much they cost right next to that. The best way is to see what a player's $/point value is and if it is worth the gamble given how much he might cost.