In the field of mental health, I'm a layman. It's that simple. However, to my untrained eye, there are several issues at play here*:
1) Mental illness comes in shadings and degrees. It's not like a broken leg. Mental illness often (though not always) creeps in a little at a time, and the patient may or may not notice the symptoms.
2) Many mental illnesses are treatable, but from what little I know of the meds involved, they all seem to run the risk of allowing the patient to believe that he or she no longer needs them. Patient feels better, thinks problems are gone, stops taking meds.
3) Where this really gets sticky is where science meets policy. Someone who seems just a little "off" might be diagnosed as having some minor degree of mental illness (science). Should that person be barred from owning firearms (policy)? Where should the two intersect? At what point and under what circumstances should mental illness result in the loss of this individual, fundamental right? Early-stage Alzheimer's? Mid-range OCD? PTSD? As a matter of policy, do we leave the law such that only involutary commitments count against one? Or do we start including voluntary commitments? Wouldn't that discourage folks from getting treatment that they need (in particular, veterans with PTSD come to mind)?
(* = I invite correction on these issues from those who know better than I.)
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.