Should I consider martial arts if I want to gain more muscle mass?

Not necessarily. Any sport to do with continuous hard exercise to the muscles (weight lifting) and the right food and supplement could provide the tool to gain muscle mass.
Yes and no. There is no doubt that martial arts will increase your strength, flexibility and balance, however you are not likely to "bulk up" like if you were a wt lifter/body builder. It depends on what you are going after. A martial artist's body is strong, but not huge like a body builder's. This is because speed and cardio tend to decrease if you are really muscle heavy!