There are several female GHT x 1/2" MPT fittings on ebay in brass as well as the link I posted in the comment. I've never seen any in stainless - of course that doesn't mean they don't exist - but I'd imagine it's a very niche product, and hence expensive. Given that brass garden hose fittings are usually acceptable, GHT stainless fittings not a high-volume product.
An alternative is to use a stainless GHT x barb bitting and a short length of 1/2" hose with a camlock on the end. On one end of the hose is female GHT x 1/2" barb, and on the other, a stainless 1/2" barb x 1/2" MPT and then the camlock (or use the barbed camlock fitting to avoid a big lump of hardware on the end.) fittingsandadapters.com lists a stainless Female GHT x 1/2" barb for $35.70.
If you hadn't already got the Therminator, I would have recommended going for the chillers at Dudadiesel - you can get them with 1/2" MPT threads on all 4 ports, avoiding the GHT problem entirely, but I imagine you don't want to hear that now!