Since you're making a lot of efforts to put away that Window dependency, I wonder why you don't provide a Gosu::mouse_x and Gosu::mouse_y methods, that are not related to a specific Gosu::Window. It seems that Gosu will not allow multiple windows anyway, and you can always think this request as "the client mouse", whatever the Gosu::Window would be active.
I was thinking about it, but I don't think I am ready to give on multiple window support just yet :/ I'll keep it in mind - it's a bit annoying in my own code as well.
I don't want to look like I'm pushing it or what, but did you have any clue about the mouse issue ? Because I'm planing to create a Theme Park like game and of course the mouse will be used a lot, and I really want to use the last version of Gosu for that. Also, I have some french tutorials to write, and I would like to use the last gem also.
If you want, you can try to replace Gosu's embedded SDL2.dll with one from the page above, and re-run your game - with some luck, that will already improve the situation.
