The recipe is not patented. This would force them to disclose it. The same goed for WD40. It's not patented because they don't want to disclose the formula.
Not only does the GPL seems like an extraordinarily poor fit for a recipe, recipes are generally considered to not be copyrightable in the first place.
"...Though the company removed cocaine from the carbonated concoction over 100 years ago, coca leaves are actually still used to flavor Coke. The soda brand has an exemption with the government allowing them to import coca leaves into a decocanization plant in New Jersey where cocaine is removed so the leaves can be used in Coca-Cola for their natural flavors."
The ingredients for the Coca-Cola flavor are listed as "natural flavors". That's the extent of disclosure.
Trade secret just means that they don't share the recipe and make efforts to keep it secret. Calling it a "trade secret" is really a marketing strategy as much as anything else. Just like the rumors that coca leaf extract are an ingredient... something that is "neither confirmed nor denied". A marketing strategy.
I think that it's basically flavored with vanilla, cinnamon, a few essential oils (orange, lemon, lavender), and some spices (nutmeg!). One you start adding that many flavors to a recipe, the exact flavors are no longer important, and I'm sure you could replicate something similar at home if you were willing to do enough experimentation. There are a few recipes online that you can use.
Also the proportions. While (in the US at least) manufacturers are required to list their ingredients in the order of greatest proportion to least proportion, they are not required to disclose what those proportions are, which also adds some difficulty to reverse-engineering the recipe.
I think the biggest catch is being able to throw potentially thousands of different substances under the generic “natural flavorings.” Read the Coca Cola ingredients list and you certainly don’t see “coca leaf extract” on it.
