I had a blowoff tube from my carboy into a bucket of starsan solution.
When I dropped the fridge to layering temps, the starsan siphoned back into the carboy. I estimate that there was about 1/3 gallon of starsan siphoned into 5.5 gallons of beer.
Is this batch a goner?
That much StarSan solution, IMHO will likely hurt the yeasties, a lot. I'd hate to find out two weeks later that it did. Sorry bro.
i wouldn't worry about it.
StarSan can be used to wash yeast for reusing yeast batch to batch... you'll be fine.