If you don't want to use the line ssh firstname.lastname@example.org 'DISPLAY=:0 retropie' and still want to run my retropie script through SSH but NOT inside your established connection, then:
Do nano /usr/bin/retropie or nano /usr/bin/gamepad
And add this line below #!/bin/bash or wherever it works for you.export DISPLAY=:0
Sometimes emulationstation crashes but I don't want to go that deep.
My job is completely done here. Have fun and stay safe.