Tamiya does a green modelling putty in a tube. Is this GW stuff just a repackaging of that?
I don't believe it is. The Tamiya, I think, is a sticky putty that is great for dealing with larger, mechanical model types such as planes and tanks, where you can fill the gap and then sand it smooth. It's a pain in the rear to sculpt anything with it, and the modifications to a mini I made (hood and mask to an overcoated figure for a scifi game) weren't difficult ones to make, but took forever to look right because it's to soft early on and it oozes a lot until dry because it isn't really intended for that kind of thing. It's almost like spackle, save a lot stickier and stains things, but is night indestructible once hardened.
Liquid Greenstuff, from my experiences is more of the texture of a slightly thicker liquid paper. It's decent for filling small gaps and imperfections, but the container dries out quickly...it's also rather interesting for making nurgal/plagued/zombie conversions...
Normal greenstuff is keadatite, and is a two part epoxy that can work well for gap filling and works extremely well for softer detail work, brown stuff being a counterpart that works well for the harder detail work...