Are you thinking of the Feingold diet which eliminates artificial food dyes, flavorings, preservatives, Aspartame and, initially, salicylates? The place to go is the Feingold Association www.feingold.org . There's also a Yahoo group http://health.groups.yahoo.com/group/Feingold-Program4us/ and a Facebook page. So worth it.
GAPS is a digestive tract healing protocol. If you're not really sal sensitive but instead are reacting to what's not broken down properly--it makes sense that GAPS would fix that.
Feingold removes sals.
We've done both. We did GAPS several years ago (2005-ish) and weren't even aware we had issues with sals. In 2007-ish, we did Feingold and found we had hellacious issues with sals. We're now going back to GAPS (for my daughter born in 2008) but we were doing it to address my youngest daughter's issues that are not related to sals.
People wind up on GAPS for quite a long time as healing the gut isn't really a "few months" thing. I'm also hesitant. We did several things back in 2005 and it was hard to know what caused what results. I've only seen/heard a few people that rave about GAPS and I wonder how many others there are who may just not have been able to stick to it long enough. But there are people that have done it even a short period of time and seen changes in their childrens sensory issues, etc. So it clearly does something positive.