I have a glass bottle with a ground glass stopper ...

I am, by no means an authority on this, so I'm just making a guess.

I'd try a few things. First, maybe get some WD40 and spray it around where the stopper meets the bottle. Let it soak in for a while and see if that loosens it up. If that fails, auto mechanics have a product that's meant to loosen nuts and bolts on car engines that are stuck together over time. Should be easy enough to find at an auto supply or even like Walmart.

Now, the caution with both of these is, I don't know id these products will damage the glass in any way. I wouldn't think so, glass is pretty tough stuff, really. Though fragile as far as brealing and chipping, it's pretty darn sturdy as far as other kinds of damage. The harm I'm thinking of is in the nature of clouding or discoloring.

If the above doesn't work, try this.

Run hot water from the tap, the hotter the better (do not use boiling water as it could crack the glass), around the neck of the bottle. Avoid getting it on the stopper.

That might expand the bottle neck just enough to loosen the stopper.

Good luck.

1. 
   Put the bottle in hot water and hold an ice cube on the stopper. the bottle should expand but not the stopper and you may be able to pull it out easily.
2. 
   Soak the entire bottle in water. There could be a sticky substance holding the stopper in and the water could dissolve it.