Doing a chargeback can have negative ramifications, so leave this as a last resort - especially since it sounds like that would be against Steam, not Wonderstruck. Some companies will stop doing business with you if you do chargebacks. I don’t know the Steam policy on this.
If the GC purchase is through Steam, you should contact their support before going through with a chargeback request. Note, though, that Steam seems to think 14 days is the return policy for DLC, but I don’t know how the weird DLC/subscription thing Boundless does will work for that.