Well its not just kills and objectives. You need to average about 100 cs every 10 minutes or more. For the 1st 10 minutes you will probably be under but if you focus farming and take gromp/krugs and raptors/wolves whenever you can you can easily get to about 230 cs by about 23 minutes. You also need high kill participation. You don't just want kills you want assists as well. Having 20 kills and 4 assists in a 50 kill game means you missed out on 36 kills worth of gold. But 500 gpm is a bit silly number to go for. People in the lcs and high challenger players do not get that number regularly. That mean's in a 28 minute game you have to have made 14k gold and in a 36 minute game that would be 18k. The issue with this is the only real way to do that is for you to be 1v5ing since the enemy team would need to consistently reset their gold value and feed you kills as well as allow you to cs and solo take towers.
Just to give an idea I played a game as kalista the other day went 15-3-4 and had 252 creeps. It was a 35 minute and 47 second game. I only averaged 487 gpm. Yes my cs was pretty bad but that is because we grouped early because their enemy team forced us to and my leblanc was double distorting waves for some reason to clear them so I couldn't get much cs other than taking camps. Even qtpie and nightblue3 both of which are pretty adamant about objectives, farming and getting kills only get 500gpm or more maybe once every 10 games.
TL;DR That 500 gpm is really only obtainable on games where your entire team is dominating(so the games do not last long) but the enemy team still resets their gold value and you get fed. This is not a GPM you can average.