Standing on Hwy 42 ...looking north at Hwy 623..... it is gravel
Standing on Hwy 667 ....looking south at Hwy 623.... it is gravel..
Using Google maps, I find if you can not place the little yellow man on the road you that you are "zoomed into", then that road is 99% of the time gravel.
As for Hwy 623, I can not get the little man on Hwy 623 in the area that you are speaking of, so I would say it is gravel based on looking at the views I explained above, however, I must say it sure looks like a curvy road that I would want to try out.
I'm certain you most likely make it all the way through, but you would have to use 2nd & 3rd gear most of the time, and stay at a slower pace.