If it's something you really want to save you could always add more wort/sugar. Considering you added the 1 gallon of original beer, there's probably some yeast still in there. Then, if you're satisfied that it ferments some more, taste it and see if it's worth saving.
If it's been sitting around without a functional airlock and has mold growing all over it I'd be inclined to just toss it though. I may have 10 carboys of sours/all brett beers sitting around at home, but I personally prefer to deal with them in a more controlled manner than this.