SwiftUI - Increment number on long press gesture, then stop once gesture is released

Two things:

1. You don't stop the timer at all, to stop it, you have to invalidate it (as Leo mentioned). You can do that with self.timer?.invalidate().
2. The .onEndedof the LongPressGesture will be called when the LongPressGesture has been recognized by pressing the button longer than the minimumDuration time, it does not handle the event when the button will be released. So you don't want to stop the timer on .onEnded.

Combined LongPressGesture and DragGesture

I tried your code and an approach could be to use the LongPressGesture to start the timer and a DragGesture with a minimum distance of 0 to recognize the button release.

This would look like:

struct CustomFoodItemView: View {
    @State var foodName = ""
    @State var proteinAmount = 1
    @State var carbAmount = 1
    @State var fatAmount = 1
    
    @State private var timer: Timer?
    
    var body: some View {
        
        // a drag gesture that recognizes the release of the button to stop the timer, set minimumDistance to 0 to ensure no dragging is required
        let releaseGesture = DragGesture(minimumDistance: 0)
            .onEnded { _ in
                self.timer?.invalidate()
                print("Timer stopped")
            }
        
        // a long press gesture to activate timer and start increasing the proteinAmount
        let longPressGesture = LongPressGesture(minimumDuration: 0.2)
            .onEnded { value in
                self.timer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { _ in
                    proteinAmount += 1
                })
                print("Timer started")
            }
        
        // a combined gesture that forces the user to long press before releasing
        let combined = longPressGesture.sequenced(before: releaseGesture)
        
        VStack{
            
            VStack{
                Text("Food Name")
                TextField("", text: $foodName)
                    .multilineTextAlignment(.center)
                    .border(.white)
                    .padding(.trailing, 10)
                    .frame(width:100, height:10)
            }
            HStack{
                Text(String(proteinAmount) + "g")
                    .frame(width:50, height:50)
                
                Image(systemName: "plus.circle.fill")
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 30, height: 30)
                    .foregroundColor(Color("SuccessButtonColor"))
                    .gesture(combined)
            }
        }
    }
}

Please see the gif with the the few items from the code snippet (the number increases after starting the long press and stops when releasing the "+" button):