Raspi zero w gpio gui output

Hi all!

I've set up my new raspberry pi zero w
It's currently booting up with Kali
Soldered my GPIO pins to it and attached my 3.5" touch screen
Using XFCE4 and slim as display manager.
The screen works great when connected to GPIO but only on CLI
When I type startx to get the GUI loaded, it only comes out of the HDMI to my monitor
It's still displaying things on my touch screen, so it's still there and touch even works but it just doesn't display the gui on my touchscreen (using GPIO).

Anyone have any ideas on this?

Cheers!