No start

All "crank, no start" conditions are approached in the same way. Every engine requires certain functions to be able to run. Some of these functions rely on specific components to work and some components are part of more than one function so it is important to see the whole picture to be able to conclude anything about what may have failed. Also, these functions can ONLY be tested during the failure. Any other time and they will simply test good because the problem isn't present at the moment.
If you approach this in any other way, you are merely guessing and that only serves to replace unnecessary parts and wastes money.

Every engine requires spark, fuel and compression to run. That's what we have to look for.

These are the basics that need to be tested and will give us the info required to isolate a cause.

1) Test for spark at the plug end of the wire using a spark tester. If none found, check for power supply on the + terminal of the coil with the key on.

2) Test for injector pulse using a small bulb called a noid light. If none found, check for power supply at one side of the injector with the key on.

3) Use a fuel pressure gauge to test for correct fuel pressure, also noticing if the pressure holds when key is shut off.

4) If all of these things check good, then you would need to do a complete compression test.

Once you have determined which of these functions has dropped out,
you will know which system is having the problem.

There's two good suspects. GM is famous for their anti-theft systems keeping owners from driving their cars. That system is part of the starter system, but it's in the starter relay's control circuit. Listen very carefully under the hood when the problem is occurring and a helper turns the ignition switch to "crank". If you hear the light click of the starter relay, the anti-theft system is not causing this problem. There may be other things clicking at the same time. To avoid confusion, put your finger on the starter relay and feel for the click.

If the starter relay is clicking, the more likely suspect then is the neutral safety switch. If you can shift to "neutral", then the engine will crank, suspect this switch.

I guess I misread this Doc. I thought it was a Crank, no start. There will be a flashing security light if there is an issue with Passkey.

What does that mean? Is the starter engaging and rotating the engine?

I'm still not real clear on what you have. What exactly is the starter doing? Is it grinding? Is it spinning free wheel without engaging the flywheel? Please be more specific. Things are getting lost in your terminology.

Sorry about butting in earlier. Didn't mean to do that.

What about a cracked flex plate?

If your starter is really acting that way, then there isn't much question that you have a bad starter and possible flywheel damage. I really think you should have a professional look at it to verify that. A lot gets lost in translation.

No problem Doc. You read the question better than me.

Tye flywheel needs to be checked while the starter is out. Those bench tests of the starter aren't always conclusive because they have no load on they starter.