If you are using Windows, then your easiest option would likely be gogcli.exe.

https://www.gog.com/forum/general/gogcli_gog_client

I've used that and gogrepo.py, and created GUIs for both.

https://www.gog.com/forum/general/gogcli_gui_frontend_downloader_and_validator

Three other programs of mine can be found via the following thread ... two of those for gogrepo.py.

https://www.gog.com/forum/general/gogplus_download_checker

Python being installed is a requirement for gogrepo.py.

https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e