been thinking about a tune for my 07GXP with 66K. But probably just the GM version so I can pass california smog. thoughts?**Edit**
@Z06-GXP's suggestion about the timing chain makes a LOT of sense - much more so than a head gasket. You might start by pulling the timing chain cover and seeing how far off it is (assuming that's the issue). It's *possible* that the chain can jump without actually bringing the valves into contact with the pistons if it didn't skip too many teeth.
If your gasket goes and the engine overheats - which will happen very quickly - the resulting warpage in the head could very easily affect multiple cylinders. The pattern you mentioned indicates that there's a serious breach near cylinder #2. If it's bad enough to actually get ZERO compression, it would have to be a very large break.
Normally, for zero compression, I would suspect a valve (or even a loose spark plug). But, you'd have to have at least four bad valves on different cylinders in this case. And, a stuck valve wouldn't give you 85 or 100 psi. I'd bet a beer that it's the head gasket.
While you're in there, may as well do some ARP head studs.
Regarding tunes on a high mileage engine: I've been running RPM's stage 2 on my 105k mile LNF for awhile now with no issues. 165psi across the board.
Since it's easy, you may as well go ahead and do the oil test. Add a teaspoon or so of oil to each cylinder via the spark plug hole before running the compression test. Also, on the off chance this isn't obvious, compression numbers will be way off if the test is performed on a cold engine, and the gas pedal has to be on the floor when performing the test in order to keep the throttle open. Don't worry about fuel as the ECM recognizes this as a command to open the throttle while not injecting any fuel.
The oil in the cylinder will also give you more accurate numbers when testing a cold engine.