You want to disable the ingame HUD and use a different one? In that case, go to “Graphics”, then look to the right, there are the HUD options, I can’t recall what the exact option was, but there are a few options regarding the HUD.
If you want to disable the original GP4 HUD, open the f1graphics.cfg using the notepad, then find: 1 ; TV style overlays on track cameras. and change it to: "0 ; TV style overlays on track cameras."