Planning on spending time in both areas. In Missoula we’re staying at an Rv park off the 93 just north of I90.
Any recommendations for long gravel rides mixed with road is OK.

Bozeman....any not to be missed suggestions?
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Bozeman - even if you can't ride the event, the route is posted and with some modification, it is fun

It weaves around a little more than necessary to get to 55 miles.

I'm not studly enough to ride it but Gravely Mountain Road is incredibly scenic and I always thought it would be a great ride if I ever got in good enough shape. I've attached a google maps route from Ennis, MT and ending at Henry's Lake, ID with a little bit of the GDMBR at the end.,+MT+59729/Henrys+Lake,+Idaho/@44.7784076,-111.8453026,11.32z/data=!4m34!4m33!1m25!1m1!1s0x535a9d823202ac4b:0xa10f665d7c2bcc6a!2m2!1d-111.7318796!2d45.3491133!3m4!1m2!1d-111.7657076!2d45.2892315!3s0x535079c85ee535b5:0xe283d9c68395e634!3m4!1m2!1d-111.8670184!2d45.1199192!3s0x535062c0f89da4b5:0x91debf21609e927!3m4!1m2!1d-111.8731592!2d44.9706772!3s0x5350f399ae82d195:0xeef198c4f7365c69!3m4!1m2!1d-111.8675177!2d44.8307048!3s0x5350e5568482104b:0x80b3e568370325f!1m5!1m1!1s0x5351a886526a9ffd:0xa15d8c9c93ff2306!2m2!1d-111.4016815!2d44.6536978!3e0?hl=en

You could shorten that ride some with a little creativity. 
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Thanks for the recommendations. Regarding Island Park to Ennis. We spend a lot of time in IP and love the gravel roads there. Red Rock Pass is one of my favorites. I’ve ridden the road around Henry’s and do a loop that takes me up the 287. Been to Ennis and I had high hopes for the Gravelly road but it was very rocky and washboard, not fun. Did find an awesome ride out of Alder (near Virginia City). Pavement around Ruby reservoir turns to a beautiful dirt road and goes all the way to Dillon😀
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