You can use the open source command-line utility
drive to back up your Google Docs, Google Spreadsheets etc. by exporting them and all other files in Google Drive by downloading them.
This is my favourite solution because it can be fully automated. However, "it probably doesn't work on Windows" [source], so may not be suitable for your case.
Installation under Linux is simple because packages are provided – instructions here. Before the first usage, go to the local target directory and configure access to your Google Drive with:
Then, to back up one specific directory completely:
drive pull -export=docx,xlsx,pptx,svg "directory name"
-export option, Google Docs & Spreadsheet files would end up locally just as links in
.desktop files. You can configure exporting to other formats as well [see].