Occasionally my boyfriend gets very defensive at certain things I say because he takes something I say different than I meant it. And, he got defensive when I mentioned his gambling with, I sure wouldn't give that casino any of my money?

If you think he gambles too much, now ... Don't plan on him changing his behavior if you marry.  Gambling can be an addiction as much as alcohol, cigarettes, or drug use.

-Being defensive in the way you expressed specially with the phrase "Because he takes something i say different than i meant it" that could be from a very automatic psychological reaction from the person's mind. I make an example to make you understand what i mean. "A person grow over their 20's, grew in an environment & faced people that they usually criticized him or almost we can say they blocked his words & made him shut, when  he wanted to say, share or express something or give out an idea. Now he grows,he was a child, he couldn't completely defend back & that behave stays in his mind as he grows, then somewhere in his life, on a topic, you wanna open up a simple discussion right after he said something, you never attend to criticize or question him, but just a friendly discussion over what he said, but he won't understand it that way, he takes it as criticize, he wants to defend & fight back & makes you not say anything about what he said & might turn so angry as well & ask you "why you always want to question or talk about something he said" & he will directly take the meaning of your action as something that you never meant it" these defensive behaviors usually has a background memory in it. If that would be the case, you need to understand it without bothering, & try to cope with it & makes it under control. For the people who take such things with their mind, it takes time to heal it up & makes it fade away.

-Check out the way you express the matter. You should talk politely, calmly & ope up a discussion, don't order but request, ask & bring reasons & be fully reasonable.

-Gaming can be addicting so strongly, you need to slowly talk about it & makes him to stop spending so much money on it but not to cut it forever, decreasing the time while to while is the best way to stop doing it.

-But you speak about "My money" is he taking the money you earn to spend on his gaming ? Or it's the money he earns & because you guys are together you count it your money ? (just a thought of if that's the case you need to know that he's free to do anything he wants with his money)

-Then, remember to not talk to him like he's your prisoner & he need to listen to you without deciding on his own.

-If none of these would be the case, surely he has a high problem that the only thing can make it done is calm & mature discussion without pointing out the fault or trying to question anyone, without making sides & without making the matter a battle to win. But only in purpose of the sake of partners, in love of each other & each other's life.