There seems to be security concerns when first trying to execute flameshot gui using hotkeys in GNOME
this gist gives the answer.
You need to create a script with particular variables that GNOME doesn’t pass and then call flameshot gui
So first create the script below ; call it flameshot.sh
Make sure it has correct permissions : chmod +x flameshot.sh
call this script in gnome keyboard hotkey configuration.
script content
#!/bin/bash
export DISPLAY=:0
export WAYLAND_DISPLAY=$WAYLAND_DISPLAY
export DBUS_SESSION_BUS_ADDRESS=$DBUS_SESSION_BUS_ADDRESS
/usr/bin/flameshot gui