A GPP is a tournament-style contest in which the prizes are guaranteed, regardless of the number of entrants. GPPs can be single-entry or multi-entry.
Single-entry means the odds of winning are equal among all participants since the amount of entries is capped at one per player.
Multi-entry means each player can submit as many lineups as they'd like for that particular contest.
Lineups entered in these types of contests are usually filled with more upside and risk. There is less "chalk" in these lineups since participants are trying to create unique lineups in order to win big.
Tip: A "chalk" player is a player with a high ownership projection (often based on the player having a great matchup for that contest).