The microscopes in question seem to be a little on the "rare" side. This doesn't mean they are worth anything, in fact, far from it, given their age and condition, however they are a little difficult to research, and very little information was obtained from an hour or so of google trawling. 

What I did eventually discover about "GS"  (Gillet & Sibert) microscopes of this vintage,  is that despite having a  London makers mark stamped on them they were built in Neilston in Scotland (so locally made.. since I live in Scotland), and were popular mainly due to their robust construction and fine optics. 

They even made a similar model for export, with a 12V input to allow the use of a car battery to light the bulb. This was intended for use in labs and doctors surgeries in remote rural communities throughout the world.

See here for the most recent Gillet & Sibert "tropical" model, which is also no longer in production.

http://www.elcomatic.co.uk/microscope.html

Light microscopes are fascinating devices, if you haven't played with them much, I suggest you remedy that as soon as possible.