To start any X application on the active display using ssh session fist you are need export environment variable with display number and the execute application binary.
$ export DISPLAY=:0 $ firefox
Pay attention, display not always has number 0, try 1 or more if it doesn’t work. And of course, you must run it from a user that have an active X session.
One line variant.
DISPLAY=:0 nohup gedit