Most shops do guarantee their work for a reasonable amount of time which varies from shop to shop and details involved!!... As in what caused the failure!.... However some may not!... So no, the mechanic does 'not have to' warranty the work!... That's usually not good business not to though!
Depends on the shop- any shop worth their salt will warranty and stand behind their work. Most "shade tree" and 'on the side' mechanics don't. That's the difference between a real shop and someone who pretends they are.
Just stating the facts there ABR.... Maybe the BBB is a better place to complain!.. Do 'you' know the details involved here?????... I don't.!!... You must know the shop that's in question since you say they pretend to be a real shop!
Give us THE ENTIRE STORY , ...........IF there were other issues that CAUSED the gasket to leak , and you only gave them permission to 'band-aid' the situation , then , no , other than the guarantee that they performed their work correctly.