Your chances are ok if it's a weekday and you arrive before noon. If it's a summer weekend, your chances are slim.
Check to see if any events are happening in the city of Gunnison and Montrose (bike rally, marathons, etc) During the week you shouldn'thave an issue.
When Ive gone its always been completely full. People drive around campground all day looking for a site. Last time I got asked multiple times if I was getting ready to leave so they could have my reserved spot.
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