Re-downloading GameGuru wouldn't bring back all the assets, he would still need to download the store bought assets. But yeah, the best way to do it would be to delete and reinstall, theoretically he could delete all the entities then use steams file verification, but I've found steams verification actually takes longer than a full install. Either way, he would still need to download purchased assets again afterwards.
What I do if I want to organise my entities is, to create a single folder called custom, then inside that add folders for buildings, boxes, characters etc. Then copy the entities into the relevant folders, yes I know it means I have duplicates, but it also means I can find them easier so I can live with it.
I also create a project folder with a similar folder structure, and copy models i'm going to use in my project into there before placing them, that way if there's ever anything missing from the standalone I can just copy that folder over and skip all the files that are present.
i5, NV960 2GB, 16GB memory, 2x 2TB Hybrid, Win10.
i3 , Intel integrated graphics, 6GB memory, 512GB Generic SATAIII Win8.1.
Intel Celeron (duel Core), Radeon integrated graphics, 4GB memory, 180gB Generic SATAII, WinVista.
Q6600, Intel integrated graphics, 8GB memory, 512GB Generic SATAII, Win7.