Some of the most vicious arguments I've seen on BF have been chain checkers vs. rulers. That's even more polarizing than 1x vs. doubles vs. triples, rim vs disc, or tubeless vs. tubes.
ANYWAY, regardless of what method you use to measure chain wear...
650 miles might be on the low end but I find that gravel wears out chains (and cassettes and chainrings and brake pads and rims) SHOCKINGLY faster than on pavement. I'd say about 3-4x as much wear.
And on wet gravel? Forget about it. Might as well take a sandblaster to your drivetrain.
That's the main reason I don't ride my geared bike on gravel much in the winter any more. I got tired of constantly replacing parts.