Rotate with DragGesture

You need to understand the how to calculate the rotation angle before implementing the rotation in DragGestureIn DragGesture, you need to calculate the rotation angle yourself.

The figure shows the idea needed to find the rotation from which can be obtained with location:CGPoint from DragGesture.

Blue is the coordinate system of DragGesture, and orange is the coordinate of the rotating body It is necessary to find the rotation angle θ in the system.

the point P become (v.location.x - self.length / 2, self.length / 2 - v.location.y) with them and the diameter of the rotating body self.length.

And the rotation angle is θ=arctan(v.location.x - self.length / 2 , self.length / 2 - v.location.y) * 180 * π (θ < 0 ? θ += 360).

Rotation angle of the object = Rotation angle of the gesture

This is an implementation method where the angle is reset each time the object is rotated.
We use atan2 (arc tangent) to find the rotation angle θ and pass it to rotationEffect.

//
//  ContentView.swift
//

import SwiftUI

struct ContentView: View {
    @State private var angle: CGFloat = 0
    @State private var length : CGFloat = 400
    
    var body: some View {
        Image("ice")
            .resizable()
            .aspectRatio(contentMode: .fill)
            .clipShape(Circle())
            .frame(width: length, height: length)
            .rotationEffect(.degrees(Double(self.angle)))
            .gesture(DragGesture()
                .onChanged{ v in
                    self.angle = atan2(v.location.x - self.length / 2, self.length / 2 - v.location.y) * 180 / .pi
                    if (self.angle < 0) { self.angle += 360 }
                }
            )
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Rotation angle of the object += Rotation angle of the gesture

I'll also show you how to implement it by adding a rotation angle each time you rotate the object.
atan2 (arc-tangent) for the starting point to find the difference between the rotation angle and the rotation angle θ of the point being moved.
When the rotation is complete, save the final angle in lastAngle and add it to the rotation angle difference for the next rotation.

//
//  ContentView.swift
//

import SwiftUI

struct ContentView: View {
    @State private var angle: CGFloat = 0
    @State private var lastAngle: CGFloat = 0
    @State private var length : CGFloat = 400
    
    var body: some View {
        Image("ice")
            .resizable()
            .aspectRatio(contentMode: .fill)
            .clipShape(Circle())
            .frame(width: length, height: length)
            .rotationEffect(.degrees(Double(self.angle)))
            .gesture(DragGesture()
                .onChanged{ v in
                    var theta = (atan2(v.location.x - self.length / 2, self.length / 2 - v.location.y) - atan2(v.startLocation.x - self.length / 2, self.length / 2 - v.startLocation.y)) * 180 / .pi
                    if (theta < 0) { theta += 360 }
                    self.angle = theta + self.lastAngle
                }
                .onEnded { v in
                    self.lastAngle = self.angle
                }
            )
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}