The quote in this meme is not “A good guy with a gun can always stop a bad guy with a gun”. The fact that the Reagan assassination attempt was not foiled by the secret service doesn’t refute that good guys with guns stop bad guys with guns in some cases. Perfection is rarely obtainable in human affairs, especially when it comes to preventing crime.
You could argue that the quote is wrong because the would-be murderer was subdued by a hit to the head rather than by being shot. But that only happened after he got out six bullets and had to reload.