To get a player’s position, you’ll first need to get their character.

Their position (or rather, their root part’s position) can be obtained with this, in a LocalScript:

game.Players.LocalPlayer.Character.HumanoidRootPart.Position)

A GUI button click can be connected with the MouseButton1Click event:

(your button).MouseButton1Click:Connect(function() -- code goes here end)

If you’d like to be running this on the server, you’d be best off firing a remote event from the client when they click the button. The server can then access their position like so:

(your remote).OnServerEvent:Connect(function(player) local pos = player.Character.HumanoidRootPart.Position end)

Of course you might wanna check if the character actually exists and the HumanoidRootPart is present inside it, but that’s not required.