open GpxPatch... because... you have GpxPatch, right?
go in tab GpxInfo Tab.
click on top the icon to Add new panel.
Enter the name of panel. click OK.
it s opening the window to set your panel.
in bottom left side can see Add Info Items button. click on it.
you can select all infos to add to your hud.
Can select gap front - gap behind.
select gap front first. Set the coordinates it appears, and the filter to apply (which camera, which track, if just in race or qualify too, just in pitlane etc...).
If you want see both set as mine the coordinates : gap front : 535,700 . gap behind 635, 730 . Font ATL 2004 .
then confirm with ok.
lunch the game by gpx patch.
all done.