GAR is how many more goals a player scores over a replacement level player, a replacement level player is anyone who has an average amount of impact on the game and is very interchangeable, GAR helps to determine who’s goals are at a higher impact then replacement level
WAR is the same but with their affects on wins
But, my point is (pun intended this time), what is a "replacement level player". That's not a real thing you can measure. Hence an estimation stat. Rather use a stat that relies only on results.