SwiftUI - How to add foreground linear gradient on image

That's because foregroundColor wants a Color, but LinearGradient is a struct that conforms to the protocols ShapeStyle and View.

If I understand you correctly you want to fill the intransparent area of an image with a gradient?

ZStack {
  Color.white // For the background. If you don't need a background, you don't need the ZStack.
  LinearGradient(gradient: Gradient(colors: [.green, .blue]), startPoint: .top, endPoint: .bottom)
    .mask(Image("AssetWithTransparency")
      .resizable()
      .padding()
      .aspectRatio(contentMode: .fit))
  }.cornerRadius(15)

The result looks like this:

Agree with @RyuX51's answer and it's working well. But some how size and alignment of my image got changed. Because LinearGradient's frame isn't set.
So here i came up with the solution for just applying gradient to the Image,

VStack{
    Spacer()
    Button(action: {
        print("Add Photos")
    }, label: {

        LinearGradient(gradient: Gradient(colors: [.green, .blue]), startPoint: .top, endPoint: .bottom)
            .mask(Image(systemName: "plus.circle.fill")
            .resizable()
            .aspectRatio(contentMode: .fit)
        ).frame(width: 70, height: 70, alignment: .center)
    })
}

The task here is to display gradient over an image. To display one view over another SwiftUI provides ZStack view, so, the code can have the next structure:

ZStack {
    <Image>
    <Rectangle with gradient>
}

Additionally, to make sure the image we use is resized correctly to the specified frame resizable modifier should be applied with correct contentMode:

Image("IconLoseWeight")
    .resizable()                     // Make it resizable
    .aspectRatio(contentMode: .fit)  // Specifying the resizing mode so that image scaled correctly

After all, we need to apply frame and padding parameter to ZStack so that gradient has the same size as the image.

The result would look like that:

ZStack {
    Image("IconLoseWeight")
        .resizable()                    // Making the image resizable to the container size
        .aspectRatio(contentMode: .fit) // Setting up resizing mode so that the image scaled correctly

    Rectangle()                         // Shapes are resizable by default
        .foregroundColor(.clear)        // Making rectangle transparent
        .background(LinearGradient(gradient: Gradient(colors: [.clear, .black]), startPoint: .top, endPoint: .bottom), cornerRadius: 0)   
                                        // Specifying gradient (note that one color is .clear)
}
.frame(width: 30, height: 30)           // Applying frame
.padding(.leading, 17)                  // Applying padding

Note, that we use a gradient from .clear to .black as we need a transparent gradient to make the image visible.