2015 GNC sports nutrition sales = $779 million (Source: 2015 GNC 10K; you can probably add on another $150-$200 million through franchise stores, but approximately 50% of GNC's retail sales are through its own private label products)
2015 VSI sports nutrition sales = $432 million (Source: 2015 VSI 10K; VSI's private label is substantially lower than GNC's)
2015 US sports nutrition market size (per Euromonitor) = $6.7 billion
Hard to breakout BB.com revenues, but INC Magazine suggests its about $500 million.
So, no, GNC is not "by far" the largest sports nutrition seller. Even if they are the largest - and they may very well not be - they are not a substantial part of the overall market for non-
GNC brands. MusclePharm may take a slight hit, but its wagon isn't hitched solely to GNC.