The short answer is, nothing in Gimp to set a default save/export folder. Also an image opened in 'some-location' will save/export to that same location.
For new images, the best way is add a location to the Gimp save/export dialogue. Then it is just a matter of clicking that location. Not too arduous.
Another way and not really recommended, is change the Windows Default save location. Your images will save there but so will all other applications. (At least there is a restore option for when it screws up)
The C:\graphics folder now takes the place of 'Documents'
For new images, the best way is add a location to the Gimp save/export dialogue. Then it is just a matter of clicking that location. Not too arduous.
Another way and not really recommended, is change the Windows Default save location. Your images will save there but so will all other applications. (At least there is a restore option for when it screws up)
The C:\graphics folder now takes the place of 'Documents'