The tap gesture have a pointables() method that gives you the tapping pointable -- there will be only one per tap, though you can have multiple fingers tapping at the same time. To identify the finger, you can use the Finger.type() method (after checking that the tapping pointable is a finger -- it could also be a tool). Once you have a list of gestures, you can identify the tapping finger as follows:
for(Gesture gesture : gestures){
if(gesture.type() == KeyTapGesture.classType()){
KeyTapGesture keytap = new KeyTapGesture(gesture);
Pointable tappingPointable = keytap.pointable();
if(tappingPointable.isFinger()){
Finger tappingFinger = new Finger(tappingPointable);
println("Tapper: " + tappingFinger.type());
}
}
}