So I've seen questions like this one looking for help uploading music with Google Music on Linux, but none really seem to confront the notion of solely using the command line. I've downloaded the software from Google and dug around in those files for anything useful but wasn't able to come up with any good solutions.
I think what you appear to be looking for is https://github.com/thebigmunch/gmusicapi-scripts - a set of python scripts to upload, download or bi-directionally sync tracks. This, in turn, draws from https://github.com/simon-weber/Unofficial-Google-Music-API should you wish to use these python libraries for your own projects.
From the linked answer:
Essentially the scripts:
Making this automatic would then be a matter of adding this to cron for repeat syncage, presumably once your initial upload had completed.
A thing to watch out for is excessively long sync / upload times that might lead to overlap of your cronned sync jobs.