Hello [name]! Thanks for joining the GMC. Before making any posts in the Tech Support forum, can we suggest you read the forum rules? These are simple guidelines that we ask you to follow so that you can get the best help possible for your issue.
Why cant I change the grid size on tile layers? They're stuck at 16x16 'cause the tileset is 16x16, but I want to change the grid size freely besides the tileset size.
The grid for a layer with a tileset cannot be changed to anything other than the size of the tileset being used. This makes sense, as if you have tileset that is 16x16, you can't place them on a 24x24 grid. On all other layer types you can change the grid size from the Room Controls at the top of the main editor window. http://docs2.yoyogames.com/index.html?page=source/_build/2_interface/1_editors/rooms.html
Then use an asset layer and not a tile map layer, tile maps have a fixed grid size and cannot overlap, but asset layers are like 1.x tiles and they can overlap.