What is a good thesis statement if you're writing an argumentative paper about gun control, and you were on the side for guns?

This is a good question.  Let's start with what makes a thesis good. 

A good thesis has two qualities. First, a good thesis must be debatable.  You should have no problem with this, because gun control is a huge issue and people are on both sides of the debate.  Second, a good thesis has to have good supporting points.  In other words, ask yourself why people should be allowed to have guns.  The better your reasons are, the better your thesis will be. 

Another element that makes a thesis strong is when you acknowledge the opposing view.  If you know the opposing argument better than they do and it is not a caricature, then you will make strides in convincing your reader. 

Based on these points, here is one example. 

I am opposed to guns as I can see very little good coming from owning them, but I believe that people should have the decision to own guns, as it is their right. 

This thesis will ingratiate you to those who want to ban guns.  In this way, you might be able to persuade them that liberty is more important than the hatred for guns.