How to enter GRS coordinates into CDU so that TGP can look at it.
So here's a quick example. Let's assume JTAC gives you coordinate "MM336626"
Switch to CDU screen.
Click following button to switch to GRS
Then using a keyboard enter "MM336626"
Then click this OBS
Enter a name for that point.
Click that button
Now switch knob from FLT PlAN to MISSION
Then press STEER + to desired waypoint (what ever you named it).
And now simply slew TGP to that area by pressing "CHINA HAT AFT LONG"
Now your TGP is looking to GRS coordinates.