Where is Greg after he beats the game on Wimpy Wonderland?

Answer 1

Greg is in his house playing Video Games for practically the entire quest.

He is still there after he beats the game, and you click on the monitor (TV) to beat the game yourself. Then you have to return the game CD to Rowley's house. Greg is still there when you return with the Joshie Club card to open the laundry room and turn off the power to Rodrick's room. He is still there when you leave to get the leaf blower from the garage to uncover Fregley from the snowman. You then go to shovel Gramma's driveway (if you haven't already, far left) and to play bingo at Leisure Towers, and use the classical CD to chase the teens from the Fast Mart. When you have used the antifreeze fluid for the truck (with the dog dish from Rodrick's room), Greg is still playing video games. Open the window curtain to snap him out of it.

He does finally accompany you outside, to follow Manny to the laundromat.

Answer 2

he goes to his house to play video games (open the blinds to get him up from his game)