GestureDetector onTap Card

As Card is "a sheet of Material", you probably want to use InkWell, which includes Material highlight and splash effects, based on the closest Material ancestor.

return Card(
  child: InkWell(
    onTap: () {
        // Function is executed on tap.
    },
    child: ..,
  ),
);

You should really be wrapping the child in InkWell instead of the Card:

return Card(
    child: InkWell(onTap: () {}, 
                   child: Text("hello")));

This will make the splash animation appear correctly inside the card rather than outside of it.