It’s been a while but Wal-Mart used to sell pre-paid credit cards. You just go in there, tell them how much you want to put on the card, then it acts as a temporary CC. Check Grim Dawn on Steam all the way to the point where you click Accept Payment and Purchase to get the full amount and just add that to your pre-paid CC. Or add some extra (like a straight $60) and then add the remaining balance from your CC to your Steam Wallet to use toward future purchases.
OR, if you’re patient, you could use the trading cards to buy the game during the next big Steam sale. What I did during this past winter sale was bought every single game that was available for under .25 that featured trading cards (there were a lot of these), left them all running in the background long enough to get the three free cards, then resold the cards on the market for about .15 each. By the last day of the winter sale, I had almost $15 in my wallet just from selling trading cards (gotta love mass sale inflation).
Edit: I should clarify that I also sold duplicate cards for AAA titles and some big-name games were selling cards for almost .50 a piece. Just gotta check the market and sell before the market is flooded so make sure you get a top price. Most cards will sell for about .05 to .10 for a game that’s been out a while.