I have a ContentView that has the following code:

struct ContentView: View {  
    var body: some View {
        GeometryReader { geometry in
            VStack {
                ForEach(0..<9) { num in
                    GameTiles(proxy: geometry)
                }
            }
        }
    }
}

I have second view (separate file) that I refactored out of my content view. The code is:

    struct GameTiles: View {
        var proxy: GeometryProxy
        var body: some View {
            RoundedRectangle(cornerRadius: 10)
                .foregroundColor(.blue)
                .frame(width: proxy.size.width/3.6, height: proxy.size.width/3.6)
        }
    }

struct GameTiles_Previews: PreviewProvider {
    static var previews: some View {
        GameTiles(proxy: //ISSUE: What parameter can I pass into proxy? )
    }
}

I have tried a couple of arguments in my preview to build my code, but none of them are of expected argument type 'GeometryProxy'

What can I use as an argument for my preview so my code builds?

If all you need in GameTiles is the proxy's width, why not just pass that property as a CGFloat:

struct ContentView: View {
    var body: some View {
        GeometryReader { geometry in
            VStack {
                ForEach(0..<9) { num in
                    GameTiles(width: geometry.size.width)
                }
            }
        }
    }
}
struct GameTiles: View {
    let width: CGFloat
    var body: some View {
        GeometryReader {proxy in
            RoundedRectangle(cornerRadius: 10)
                .foregroundColor(.blue)
                .frame(width: width/3.6, height: width/3.6)
        }
    }
}

struct GameTiles_Previews: PreviewProvider {
    static var previews: some View {
        GameTiles(width: 100)
    }
}

Then you can use whatever value you want in the preview.

@Roosterboy - Appreciate you chiming in! The example I posted was a very simplified version of my actual project - I'm pulling multiple views out of a GeometryReader. I should have clarified this in my original post.

@Obelix - This worked great in my project! Thank you for helping me understand the preview better :)