1989 gmc diesel 6.2 timing gear diagram?

The harmonic balancer line is at 0.

Cam gear dot at 12
IP gear dot at 6

you can see that the dots line up

There's a mark on the timing cover, and a mark on the IP itself. Line those two marks up and the engine will run as set from GM. You can bump the IP mark a dime's width(not far at all) to the driver's side to advance the timing slightly and give you a tad more power.

That's the "basic" way to set timing. To get any more advanced you need a timing light especially designed for a diesel.

Here's a picture of the two marks on a 6.2