There is, or at least used to be, an app called GPXNavigator that would let you import and follow GPX files.
I don't have an iPhone and do have a Garmin so I have no experience with it but it may be worth looking for.
GPX viewer apps seem fairly plentiful but they will only allow you to look at the map and in some cases see your location on it.
I assume that printed turn-by-turn directions will be available along with the data files at least I hope that they will be.
I do not know how it went in Colorado, but we had a lot of trouble with routing when the files were loaded into Garmins with different software revisions.
Some of our routes got pretty "creative", and the leaders had their work cut out for them.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked