A gun lock is a device which inhibits the use of a firearm, ostensibly for the purpose of safety. While a variety of forms exist, all serve the purpose of preventing the gun from being used until the lock is disabled. Requiring expensive gun locks on all firearms can be a form of gun control and an affront to the liberties guaranteed by the Constitution.
John Lott said, "... it's not in everybody's interest to have a lock on their gun. If you live in a safe area and maybe have young kids, that might be fine. But if you live in a city, even if you have kids, I don't think it's really the wisest thing to have the gun locked up, because you're not going to be able to quickly access it to defend your family. And when you compare probabilities, accidental gun deaths in the home are trivial compared to the rate at which other types of deaths occur from crimes where innocent victims are attacked and a gun would benefit them."
...the gunlock was that part of the pre-modern firearm now known as the trigger mechanism. A 19th century (and earlier) musket had three parts; a lock, a stock and a barrel - hence the modern-day phrase meaning "all of something."