From the tech support document pinned in discussions:
---- HOW TO UNINSTALL GALACTIC CONTENTION ----
First things first make sure you unsubscribe from the mod so you don't receive further updates. Typically once this is done it should remove all files related to Galactic Contention yet never sorry to double check. First you'll need to navigate to where the steam folder is in the same drive as where Squad is installed.
Go to your steam folder -> steamapps -> workshop -> content -> 393380 (This is Squad)
From there, there will be list of files with numbers (each one represents a mod). You want to delete the file that is 2428425228. That will remove GC entirely
Finally I recommend clearing cache data in the steam settings
Steam top left corner -> Settings -> Downloads -> Clear Cache
Sorry to hear that you will be uninstalling the mod, but hope you enjoyed your time while playing and hopefully we'll see you return some day!

Thank you!