20-20kHz is the limit of what most people with "good" ears can hear, but more than most people with normal or worse ears can hear. If I had to guess, I'd say that very few people can actually hear 20 or 20k, although you can certainly feel frequencies below 20Hz. Those frequencies are also pushing the limits of what speaker systems can produce, even top-quality systems.
A PZM or PCC mic would not be my first choice. They tend to have very uneven frequency response, and I've never used one that would satisfy your low self-noise requirement, especially if the self-noise of a C414 is too much for you.
Honestly, I think you just need to try a few different mics and see what works best for you. It's possible that a C414 won't be up to your standards, but on the other hand it might be perfect. You also might be fine with a normal small-diaphragm condenser like a KSM137, or perhaps something even cheaper.
In any case, in a live setting, I don't think the frequency response and self-noise of the microphones will be the limiting factor. You can easily find a mic that will be flatter and quieter than the rest of the system you're running it into. Some mics will certainly be better than others, but I wouldn't get too hung up on noise levels if I were you. I think anything reasonably quiet will do fine, as long as the frequency response is suitable. You may even find that a mic with an uneven frequency response works better for you, or helps make up for inadequacies in the PA system.
I can't say for sure what mic will work best on your gongs, but I think good large diaphragm condensers are your best bet. In my inventory, I'd reach for my C414's first, because I think they'd sound the best based on what I have. There's no way to know for sure unless you try a few though. If I didn't like the way my C414's sounded, I could try some KSM32's, SM81's, KSM109's, AT4041's, ADX-51's, or even DPA4099's, to see if I liked them better, and a few other options before I was reaching for the bottom of the barrel. I have more options than some people, since I have about 200 wired mics in my inventory, and about 80 of them are condensers. See if you can find someone in your area that would let you try a few things out. The best way to narrow down your choices will be to try the available options and see what works best.