In Blender you place both objects at the same position, then you bake the normals of the highpoly to the lowpoly, you select the highpoly first, hold shift and select the lowpoly then additionally, this way the lowpoly is the 'active'. Now you can bake 'selected to active' - that's the button beneath the big Bake button.
http://wiki.blender.org/index.php/Doc:Manual/Render/Bake
Here a detailed video (I just found it per googling):
http://www.veoh.com/browse/videos/category/educational/watch/v6379634HgSpwjCMyou only need the part of the video until 4:40 (Didn't had a look into each detail)
For the use in 3DGS, you have to mirror both models, before baking the normals.
Hope, this helps so far.