Just bought a 1999 Chevy express 1500 that would not start.

Turns out it was the Passlock control module that had fried.

Bought a rebuilt one, did the relearn procedure and it started.

But now its surging hard while at idle. Looking for correct idle?

Does it have a IAC valve?

If I add gas or change gears it will die

 

I think you do have an IAC, but you can always check by look in a part catalog like rockauto.

give it a clean and test it out

https://blog.1aauto.com/how-to-diagnose-engine-surging/