If for some reason it is not found until en route, unresponsive is easy, drop mag, clear chamber, place gun, mag, (bullet if there was 1 in chamber) into a bag and given to police (Edit, I am certified to handle guns by Massachusetts). If the pt is legally allowed to own/carry he will have no issue. If it's illegal I insured scene safety. Responsive and on scene I would quietly mention it to police in case it's not legally owned, ensuring scene safety.
En route and responsive, it would depend. I would NEVER reach for a persons sidearm in a responsive patient unless I was unable to get out of the truck and it was unsafe. Now please understand I have many hours working on disarming knife/guns with experts and I would not expect anyone to try it without 2 things, knowledge on how to disarm and a fear of your life. The safest way to be safe is to run and cover. Now I have never worked on an ambulance, I am still in class, but that's how I would handle it.
NEVER handle a gun unless you know what your doing.