I use VSL#3 and primadophilus reuteri. Gas is not an issue. I have noticed I feel worse when I stop temporarily. I have been taking these for six months.
VSL#3 has undergone some clinical trials with good results (not yet published for Crohn's), but the cost is high. There are also a few journal articles on Lactobacillus reuteri / reuterin.
There is some positive data for S. boulardii but people might be hesitant to try it because it is a yeast. Many with Crohn's (myself included) already have antibodies against S. cerevisiae, so I am waiting for more data.
Also thought I would add:
I asked my GI before I started probiotics and Omega-3's and his response was "sure, they can't hurt; anyways I take them myself"
Also keep in mind that Crohn's is very heterogeneous, so whatever works for someone might make someone else gassy...
I do not think it is fair to generalize.