It would be different for OpenGrade X,
but for OpenGrade and OpenGrade3D:
First you need a GPS receiver connected trough USB. With this you can use all feature but the automatic valve control.
The standard Ardusimple F9P is a good choice.
The valve control is connected trough a second USB port with an Arduino nano. For the PCB I recommend the Original V2 or the Kaupoi. Put a cytron, a little joystick and some indicators lights.
There is also an option to use the CaseIH/John Deere auxiliary valve but I don’t know much about it, more info here (grade control, post 426 and up)