As of now the GUI shifts left or right depending on witch vehicle you enter in my case i use AAI Chain gun car and miner,to reproduce enter a car position the gui where you want it exit...enter a different car the gui has now shifted to a new position...reposition it again (now if you enter the same car the gui stays where you put it,if not it shifts)
Suggestion add a option if possible to lock the gui either wherever the player puts it or above or to the left of the character quick inventory like in your screenshot