两件事：
1.你根本没有停止计时器，要停止它，你必须使它无效（正如Leo提到的）。你可以用self.timer?.invalidate()来实现。
1.当按下按钮的时间超过minimumDuration时间时识别到LongPressGesture时，将调用LongPressGesture的.onEnded，它不处理释放按钮时的事件。所以你不想停止.onEnded上的计时器。

组合长按和拖动手势

我尝试了你的代码，一种方法是使用LongPressGesture来启动计时器，并使用最小距离为0的DragGesture来识别按钮释放。
这看起来像：

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 longPressGestureIncrease = LongPressGesture(minimumDuration: 0.2)
            .onEnded { value in
                self.timer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { _ in
                    proteinAmount += 1
                })
                print("Timer started")
            }

        // a long press gesture to activate timer and start decreasing the proteinAmount
        let longPressGestureDecrease = 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 for increasing the value
        let combinedIncrease = longPressGestureIncrease.sequenced(before: releaseGesture)
    
        // a combined gesture that forces the user to long press before releasing for decreasing the value
        let combinedDecrease = longPressGestureDecrease.sequenced(before: releaseGesture)
        
        VStack{
            
            VStack{
                Text("Food Name")
                TextField("", text: $foodName)
                    .multilineTextAlignment(.center)
                    .border(.white)
                    .padding(.trailing, 10)
                    .frame(width:100, height:10)
            }
            HStack{
                Image(systemName: "minus.circle.fill")
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 30, height: 30)
                    .foregroundColor(.red)
                    .gesture(combinedDecrease)

                Text(String(proteinAmount) + "g")
                    .frame(width:50, height:50)
                
                Image(systemName: "plus.circle.fill")
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 30, height: 30)
                    .foregroundColor(.red)
                    .gesture(combinedIncrease)
            }
        }
    }
}

请查看包含代码片段中的几个项目的gif（开始长按后数字增加，释放“+”按钮时停止）：