A Steam user has found a workaround that allows you to install the game despite the technical issues Steam is having with the launch. I have personally verified that this works, and here is the step-by-step process:
1. Make sure that Steam is launched and running on your computer.
2. Press the Windows key and the R key. This will bring up the command window shown in the picture.
3. Replace the default text in the window with the command line as in the picture, then press Enter. Here's said command line for your convenience: steam://install/949370
Now Steam should give you the option of choosing where to install the game, and once you have you're good to go. Massive thanks to Steam user Riedy for providing us with this solution in their review on our Steam Store page! We contacted Steam about the issue yesterday, so hopefully it'll receive a proper fix soon but until then, this solution works perfectly.
Normally I wouldn't ask you guys to share this post, but it would be immensely helpful in this case as we had a troubled launch. Again, huge thanks for all of your fantastic support all along the way! SGC still has a few days to go, and we'll be there for every one of them, so we hope to see you there!