My anti virus software flags the Shadow Warrior installer with a virus having a WS.Reputation.1 signature. Anyone else getting this, and is it a false finding?

Considering that GOG are a known name in digital distribution, the file should be clean (it really isn't good business sense to be distributing viruses to your customers).

You can raise a support ticket to GOG if you are concerned, but I'd recommend contacting your anti-virus support and sending them the affected file. This way they can then look into the file and why it is being flagged as a virus, and then updating their definition files so that the file doesn't get identified as a virus in future versions.