Under federal law 18 U.S.C. § 922(d), it's unlawful to sell or otherwise dispose of any firearm or ammunition to any person that “has been adjudicated as a mental defective or has been committed to any mental institution.”
In general, this means you can’t possess a gun if you’re considered a danger to yourself or to others, or lack the mental capacity to manage your own affairs according to the Bureau of Alcohol, Tobacco, Firearms, and Explosives regulation 27 C.F.R § 478.11.
Trump needs to extend this to people with mental disorders.