Sign up for emails at every level that interest you. Something to also keep an eye on is the "Schedule of Proposed Actions" (SOPA). Use the search function on the GPNF main page to find it.
Also keep this knowledge and apply it to any National Forest you have interest/concerns in.
The USFS does work with individuals to keep them informed. You only need ask to be included on these Mailing list.
Maybe I am still missing it. I reviewed the SOPA and there is one action that discusses road closures. No numbers or other info, but they are in a watershed so my guess is it would be a waste of time to find their numbers to recommend them as trails. The questionnaire references roads recommended for closure or decommissioning. There are no specific roads recommended for those statuses. If there are none how can we give input on what we would want converted?
From Ken's response it sounds like any road can be recommended by the public for trail status?
I would simply state that all roads are important for public use and that any road that is being looked at for decommissioning be considered for a conversion to 4x4 trail use. There is a complete lacking of 4x4 trails in the GP and that is unacceptable.