In TX you would have been justifed to use deadly force to stop the theft if you believed that there was no other reasonable way to prevent the theft and if you believed that the owner of the property would use deadly force if he were in your situation.
I suspect that the cops would be pretty happy to shake your hand if you kept someone from stealing a gun. Stolen guns have a way of turning up in a way that makes cops' jobs harder.
Would I do it? Unless a life was in danger, I'd probably sit that one out. On the other hand you would want to keep a very close eye on the guy--you never know if he's bent on stealing a gun, or if armed robbery is his motive and he just didn't have a gun. Might have a box of ammo, or even a loaded clip in his pocket.