Yes, I would definitely think you need to add a culture to make a gouda, or any hard cheese. Fortunately, you don't need a specialized, packaged cheese culture. Regular supermarket buttermilk contains mesophilic cultures, and regular supermarket yogurt contains thermophilic cultures (just make sure it says on the container that it contains live and active cultures). For gouda, you will need mesophilic, so pick up some buttermilk before your next try and you will fare much better. I'm not sure about the amount to add, but I know it has been posted here somewhere. If somebody finds it before I do, please post.
BTW, what recipe are you using?