There are 2 ways of making 2d games in 3dgs:
1) Using panels and moving them based on this. An example is a project we made just recently (banana run): http://www.download.com/Banana-Run/3000-2099_4-10525812.html?tag=lst-0-1
2) Using 2d sprites in a 3d level. This way you have much more freedom, and is much easier to work with. Also this way you can have a much more "3d" feel for your 2d game.