Launch your game with GM, using the -v option, as if you were to create a log. Exit.
Launch Arcade OSD. You'll notice the option "Get mode from clipboad" is enabled. Use it, it will load the last video used by GM.
Make your tweaks. When you're done, use the option "Copy mode to clipboard", then open Notepad, CTRL+V and you'll get the relevant values to create a custom crt_range.
My advice is to only use the horizontal values, leave alone vertical porches, unless the picture is not centered.