Get back to the garage, that is not acceptable, dont repay for changing gasket again, offer to pay for any secondary problems, I.e what was causing gasket to go, but not for changing gasket again, even better demand your doh back and go to a garage who can be arsed doing it properly. It is probably the head get it checked for cracks and that it aint warped, even if it is, if it aint too bad you can get it skimmed. Also check top of block carfully and also check for trueness. Dont skim too much as you are raising compression ratio and go to far you might end up with detonation problems. Valvers need proper coolant mix all thier life to avoid corrosion problems in waterways once they get older. Think most valvers eat a gasket at least once in thier life, although mine aint yet at 86k (touch wood). Dont take any crap of that garage though!