GMRoomPack’s job is to place the things in the room, the rest is up to you.
Don’t think I still have the source files for that demo, but pretty sure I just moved the objects around based on xstart + camera_get_view_x(view_camera[0]). And if your camera rotates, you gotta do math (but also at that point you’ll probably have to switch to GUI + display_set_gui_size)