robert gentile was charged with selling a firearm to an undercover agent .
but his attorney claims the arrest was a ruse to get gentile to talk about the 1990 boston art heist at the isabel stewart gardner museum .
gentile 's house was searched in 2012 and police found a list of the stolen art pieces and their estimated value , as well as police uniforms .
the half-a-billion heist remains unsolved and fbi has never come close to finding thieves .
gardner museum continues to display empty frames , and is offering $ 5million reward for the return of the works .