Can a glider be dropped from geosynchronous or other orbits and safely land?
It depends what you mean by "glider".
If you mean any craft that trades height for horizontal movement while flying without engines then yes. See the space shuttle.
If you mean a craft designed to fly without engine power while losing as little height as possible and exploiting thermals then no.
The problem is as you lose height you gain speed and when there is no significant atmosphere you have no good way of getting rid of that speed. So you inevitably hit the atmosphere extremely fast. The difficult part of coming back from space is losing speed to atmospheric drag without your craft burning up or ripping itself apart in the process.
To reenter in one peice requires thick heat shields to avoid burning up and a compact shape and robust construction to avoid breaking up.
To fly for a long time without engine power while losing as little height as possible requires a lightweight construction with long slender wings.
These requirements are mutually exclusive.
I did some quick searches to put this in perspective. Afaict a modern glider has a glide ratio of about 50, a 747 has a glide ratio of about 15, concorde has a glide ratio of about 9 and the space shuttle has a glide ratio of about 1.
That's pretty much how space shuttles and other orbiting craft work.
They are dropped from orbit, they do calculations to enter the atmosphere at a velocity and attitude that doesn't burn up the aircraft, then they glide to their landing airport.
The entire journey is controlled, but unpowered flight. The flight surfaces are there to allow the pilots the ability to reduce the velocity and attitude of descent in order to get controlled descent into the atmosphere.
So a glider can also land; assuming of course it had some way of slowing down and is sufficiently hardened and shielded against the atmospheric effects. I am not too familiar with gliders but from what I know they don't have any spoilers or other such controls.
This is more applicable to space exploration really as the problem is de-orbiting and surviving re-entry rather than gliding.
If you are in geosynchronous orbit then you have a lot of velocity, if a glider was suddenly dropped into that orbit it would continue to orbit unless some sort of propulsion was used to slow it down enough to re-enter the atmosphere.
If it was to survive re-entry from orbit it would need a Thermal Protection System (TPS), or heat shield. Without one it would fry.
Possibly you are asking what would happen if you dropped a glider from a geosynchronous orbit height but with no orbital velocity. If that happened then it would start to fall towards the earth because of gravity, and by the time it hit the atmosphere it would be travelling well in excess of Low Earth Orbital speed, meaning it would still burn up unless it had a TPS.
Can a glider re-enter the Earth's atmosphere and land? Sure. As pointed out above, the Space Shuttle was one of the best examples of this but other types e.g. X-37, etc exist as well.
Also an orbiter doesn't exactly 'drop' from geosynchronous orbit; its in an energy state which allows it to remain there perpetually; the energy must be bled off via a de-orbit burn using rocket engines to alter its orbital trajectory to make contact with the atmosphere. The burn has to be precise to place the craft on s specific trajectory to contact the exosphere at a specific angle. Too steep, the craft will be incinerated and break up, too shallow and it can skip off the atmosphere like a stone skipping across a pond.
In addition to a heat shield, the shape of the craft must be a blunt object for initial re-entry and aerodynamic enough to provide an acceptable glide ratio to a runway. The shuttle did this via its de-orbit burn while orbiting facing aft and inverted(!) thence tipping its nose forward and entering the atmosphere belly first and then transitioning to more of s traditional glider attitude once it had slowed considerably and most of the kinetic energy had bee dissipated.