WD40 is a water displacer first and foremost (I've always assumed that's what WD stands for - anyone know if this is right?). Better than nothing but it all evaporates away leaving little protection. It's also a great solvent, so good for cleaning stuff with, but can dissolve out other grease/oil that would be better off left on the chain or in bearings etc. Not ideal for bikes if it's the only thing you use, but certainly has it's uses. I don't personally find GT85 that much better for bikes if used by itself for the chain/cassette, but do use it to squirt into cable inners where it's tricky to get light oil in (which I use for preference everywhere I can easily apply it).