Use Coordinate spaces to convert tap location.

SwiftUICoordinateSpacesExample.swift

// SwiftUI Coordinate Spaces example
// Xcode 12.3
// We show two colored rectangles on screen. When any of these rectangles is tapped
// we convert the tap location into VStack's coordinate space and show a circle at
// that location.

import SwiftUI

struct ContentView: View {
    @State var locationOfTapInStackCoords: CGPoint? {
        didSet {
            if locationOfTapInStackCoords != nil {
                print("location of tap in stack coords is \(locationOfTapInStackCoords!)")
            }
        }
    }
    static let topLevelStack = "topLevelStack"
    var body: some View {
        VStack {
            GeometryReader {
                geometry in
                Color.purple
                    .gesture(DragGesture(minimumDistance: 0, coordinateSpace: .local)
                                .onEnded { (dragGestureValue) in
                                    handleTap(at: dragGestureValue.location, usingGeometry: geometry)
                                })
            }
            GeometryReader {
                geometry in
                Color.orange
                    .gesture(DragGesture(minimumDistance: 0, coordinateSpace: .local)
                                .onEnded { (dragGestureValue) in
                                    handleTap(at: dragGestureValue.location, usingGeometry: geometry)
                                })
            }
        }
        .coordinateSpace(name: Self.topLevelStack)
        .overlay(Group{
            if locationOfTapInStackCoords != nil {
                Circle()
                    .stroke(Color.black)
                    .frame(width: 20, height: 20)
                    .position(locationOfTapInStackCoords!)
            }
        })
    }
    func handleTap(at location: CGPoint,usingGeometry geometry: GeometryProxy){
        locationOfTapInStackCoords = geometry.convert(location, in: CoordinateSpace.named(Self.topLevelStack))
    }
}

extension GeometryProxy {
    func convert(_ point: CGPoint,in coordinateSpace: CoordinateSpace) -> CGPoint {
        let frameInOtherCoordinateSpace = self.frame(in: coordinateSpace)
        let dx = frameInOtherCoordinateSpace.origin.x
        let dy = frameInOtherCoordinateSpace.origin.y
        return CGPoint(x: point.x+dx, y: point.y+dy)
    }
}