I was also wondering the same thing. My boyfriend loves the Gouramis and I love my betta. I was considering buying a Blue or Yellow Gourami until I researched them a little bit more and found that bettas and Gouramis are a lot alike. I was afraid that one would kill the other due to their aggression levels or fight over surface space or something along those lines.
They will definitely fight if the Betta is a male. If it is a female it is more trial and error than anything else. Females are generally less aggressive than males but that is by no means the rule, some females can be just as aggressive. Have a back up tank ready if you try mixing because there is a high chance you will have to remove the Betta. Don't try if it's a guy. =)