Can grappa be used instead of white wine for cooking?

No. Grappa is a distilled spirit like brandy or Bourbon. It's too strong in alcohol and in flavour to substitute for white wine. It can be used where dishes require brandy say for flaming off the juices from a cooked steak. White vermouth though stronger than wine in alcohol can sometimes be a substitute for white wine particularly if you want to add a herby mediterranean taste.