Is anyone here a nutritionist or sport medicine doctor?

I have several questions about value of sport beverages for mountain bikers. There are several kinds of beverages on the market: endurox, gatorade, creatine etc...

Are these beverages actually improve biker performance? or
Are they just marketing gimmick?

Is is safe to take creatine to increase my muscle mass for biking? I really don't want to be body builder.

What nutrition supplements can I take to gain weight without detrimental consequences?