does the mechanic have to warranty the work done on a intake manifold gasket rep

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!

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.