Not a game in the traditional sense, but actually a really cool little experience! I love the idea of leaving messages for players to find along the way up the hill. Some actual mechanics would be great to maybe break up the monotony of pushing the boulder for too long, and perhaps some way of going faster/getting further would help space out some of the messages a bit better.
I will just note this maxed out my GPU, maybe 2D markers for past players would work better than the 3D models, or at least simpler models.
But yeah, a really cool way of letting people leave their virtual mark, nice one!