In theory, if it smells badly and is yellow, it contained ethanol when it was new. The ethanol is hygroscopic (it absorbs water), and the hydrated material, probably mixed with some of the constituents of petrol, decomposes. It is not related to gum formation, but it does make a claggy mess that won't go through the jet, and clogs the float bowl.
I don't like ethanol in fuel for several reasons, but if the fuel is going to be kept for a long time without airtight sealing to keep the moisture out, it is just a bad idea to use it.
