After reading your post again, sounds like tome like you had pure water in your radiator and not the 50-50 mix that is recommended. I think at 5200 ft, the boiling point of water goes down a couple of degrees so the altitude thing has nothing to do with it. I think the gurgling stopped when the car was running because the water pump was running and as the water pump is running it is forcing liquid throughout the engine and pressurizing the system, increasing the boiling point, thus eliminating the gurgle in a nutshell. Once you cut off the engine, no more water pump, the de-pressurization of the 105-110 liquid (if not a 50-50 mix) will vaporize, thus causes the gurgle. That is probably why you were losing liquid out of the reservior. I would flush the system and replace with 50-50 mix and look for leaks.

I believe with a 50-50 mix, you would increase the boiling point of your cooling water from something like 212F to 250F, not sure. Also, removing vapors in your liquid will increase your heat transfer and your temp gauge will probably read around 95 to 100C instead of 105-110 C the next time you go up that mountain

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)