The most likely issue is you missed the folder that contains the list of downloaded store items when backing everything up. Without this folder GameGuru doesn't know the items are installed and therefore starts the download again.
Unfortunately if you didn't backup this data there is no way to restore it without a copy.
I am not on a Windows machine at the moment and can't remember the path of the directory, maybe another user with GameGuru installed can tell you the path for the folder.
I think it may be something like /Files/editor/downloads and is filled with a lot of text files.
Windows 7 Pro, Intel i7 3.8 GHz (Passmark: 9021), 16GB DDR3, NVIDIA GTX 780 4GB Superclocked (Passmark: 8056)