Unfortunately you didnt tell me what kinda of lock this is.
IE 1) Trigger Lock 2) Cable Lock

Now from what i understand about them, for 1 unless you spent high end dollars on your lock its a piece of garbage anyway. Read this page

First method id try is "bumping" (this is going to be the cheapest method for you without resorting to a locksmith or destruction of the lock.

"Bumping" involves using blank keys to effectively pick locks. Watch this You Tube Video

In order for you to try this you would have to go to the place that sells your model gun lock. ask them if they have extra keys, if not i know locks that tend to be simple (4 pin locks like trigger locks) dont have that many combitnations and if you went through 20 boxes (if you are able to do this at a local store) you WOULD find a key to use that would fit yours.

NOTE if you cannot obtain a key blank @ the local shop go to the local locksmith shop, tell them you want a blank for it, see if they can provide it (i have no clue) (most likely though unless its one of those insanely weird key types)

2) Still a cheaper method would be probably to destroy the lock (irregardless of this locks price, scope out a new lock and see how much that would cost and call a locksmith and ask what picking and re key ing your lock would cost)

3) You could if in the right area of the world, obtain your own lock pick set ( a simple set will work here), and pick the lock yourself. LESSON to pick a lock you a few tools and will always be using 2 @ a time.) for 1 you need a tension wrench this is a tool that allows you to apply tension into the inner cylinder (imaging when you spin your key what way the key spins, you need to apply force in that direction) NOTE just a little bit. the reason for this is it creates a pinching zone inbetween the two cylinders in the lock (the pins are what bind them together). With the right pressure on the cyclinder with your torque wrench you can "rake" the pins (scrape them up) hoping that as they go up they dont come down. (NOTE some pins have 3 pins and fake pins and are set up in a way that makes them hard to pick try and determine which pins are longest first)

For more informaiton on this i can be contacted it should be a matter of enough rakes.

GL

Brad

Please Rate this as FixYa :)

NOTE when a lock is in the open position it is easier for a locksmith to fabricate a key :)

Go to a locksmith and ask the they make a bump (999) key for you. Now, most locksmiths will say no because they know that they can get in trouble. But it depends on where you're located. In some areas, lock bumping is okay as long as you're going to do it on your locks. So check your local laws.
Sure, you can just ask the locksmith to bump the lock for you. But that would mean that you have to pay him extra money when a blank or bump key will just cost a few dollars. If he's not willing to make a bump key, just buy a blank and make a bump key yourself.