GRAS just tells people what they're allowed to sell, but most food products have been around long enough they are grandfathered in. The bigger issue that the FDA still mandates, is that the products are prepared under cGMP standards. The standards are open, available, and not at all onerous if you intend to be an honest business.[2]
Essentially, you need to have trained staff, clean buildings, batch records, clean equipment, known ingredients (no allergens accidentally introduced), etc. Really basic stuff. Unfortunately, it's mostly self-policed until an 'incident' at which case the FDA can do an audit and raise hell.
[1] - http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/
[2] - http://www.fda.gov/cosmetics/guidancecomplianceregulatoryinf...
- The manufacturer of a dietary supplement or dietary ingredient is responsible for ensuring that the product is safe before it is marketed.
- FDA is responsible for taking action against any unsafe dietary supplement product after it reaches the market.
So, Soylent is probably considered a food since it is a sole item of a meal.
http://www.1stphorm.com/products/men/level-1
I wouldn't really consider using anything that isn't, but lots of people do every day, probably mostly from ignorance.