I had that same question myself. I don't know if RJ meant Shadowspawn died only through a Traveling gateway, or if the gholam was somehow exempted, or if this was one (of many, unfortunately) continuity errors in the book.

One other possibility is if the Shadowspawn don't die immediately, but as an after effect of passage throught the gateway. Either way, the gholam is finished, but I wonder if they will retcon this in some manner.