Gas is a common response to the use of probiotics. You are changing your gut community of organisms (hopefully for the better) and that is going to alter digestion. It can take a while to adapt, thus the strategy of starting slowly as mentioned by subzeromambo (wherever did you get that name?).
I didn't have any gas problems with VSL which may have been a clue that it was not doing anything for me.
I have been experimenting with another brand using three separate strains and am experiencing gas, although it is not bad at all. On a positive note, the new probiotic actually seems to be helping. I expect that the gas will subside with time as a different gut community is established.
My problem is that this new probiotic is even more expensive than VSL and not covered by insurance. *Sigh*