Download your GOG games offline as standalone installer files, without GOG Galaxy.  This can be useful to ensure one has a source copy, as opposed to relying on the GOG Galaxy client or other download managers. For those who are interested in purchasing SC4 from GOG (or might've done so already), it's an important consideration. It makes certain the game is downloaded in its purest form as Maxis intended, without any other superficial software layers to otherwise complicate matters. I've made a video which shows the exact process for how to do this from your GOG games library:
  (In future I might make some more SC4 and site related tutorials too.)
This explains the steps to obtain the offline installer for SC4 and also other GOG games you may have. Most games should allow this (including SC3000 and SC2000), although there might be some which are an exception. Once you've obtained the installer file for any given game, it'd be a wise move to make a backup copy on your local storage media in more than one place. An example would be on flash drives, or ideally a more robust method of storage such as an external HDD separate from your computer. Hopefully this helps to clarify and might come in useful.