Both BCAA and creatine are popular in the workout world to build muscle, especially when it comes to aerobic exercise. The amino acids categorized as branch chained amino acids are Valine, Leucine and Isoleucine. Nearly half of the amino acids derived from the protein in your diet are made of BCAAs. When you exercise, BCAA are utilized as a source of energy, helping protect existing muscle and assisting in building new muscle. They can also help prevent post-gym soreness and improve muscle recovery.
Creatine is also derived from dietary protein (found in beef, chicken and fish) and is transformed into a substance that helps your body make ATP (energy). Your muscles need ATP for muscle contraction, so making sure your body has creatine can aid in increasing your power and strength during workouts.
Since BCAA and Creatine serve different purposes when it comes to exercise, taking both before a workout can be helpful if you're looking to build muscle and increase performance.
The idea of BCAA causing weight gain came from an animal study (5) using mice that observed over eating/increased food intake and weight gain when the mice were fed BCAAs. Exercise was also not a part of this study, so it's difficult to see how this would translate to humans and play out with increased activity. If you are using BCAA for exercise benefits, there is plenty of research documenting the benefits of supplementing that way.