i am using the proprietary nvidia driver. xrandr (which is the same as xrandr --q12) can only detect the native resolution as the driver does not implement xrandr1.2 completly:

 ~ $ xrandr 
Screen 0: minimum 8 x 8, current 2880 x 1800, maximum 16384 x 16384
DP-0 disconnected primary (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-2 connected 2880x1800+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
   2880x1800      60.0*+
DP-3 disconnected (normal left inverted right x axis y axis)
DP-4 disconnected (normal left inverted right x axis y axis)

so i can only choose this resolution in games.

it would be nice, if you could use xrandr --q1 to detect possible resolutions as this shows all possible resolutions:

 ~ $ xrandr --q1           
 SZ:    Pixels          Physical       Refresh
*0   2880 x 1800   ( 975mm x 609mm )  *50  
 1   2560 x 1600   ( 866mm x 541mm )   51  
 2   2560 x 1440   ( 866mm x 487mm )   52  
 3   1920 x 1200   ( 650mm x 406mm )   53  
 4   1920 x 1080   ( 650mm x 365mm )   54  
 5   1680 x 1050   ( 568mm x 355mm )   55  
 6   1600 x 1200   ( 541mm x 406mm )   56  
 7   1440 x 900    ( 487mm x 304mm )   57  
 8   1366 x 768    ( 462mm x 260mm )   58  
 9   1280 x 1024   ( 433mm x 346mm )   59  
 10  1280 x 800    ( 433mm x 270mm )   60  
 11  1280 x 720    ( 433mm x 243mm )   61  
 12  1024 x 768    ( 346mm x 260mm )   62  
 13   800 x 600    ( 270mm x 203mm )   63  
 14   640 x 480    ( 216mm x 162mm )   64  
 15  2880 x 1620   ( 975mm x 548mm )   65  
Current rotation - normal
Current reflection - none
Rotations possible - normal left inverted right 
Reflections possible - X Axis Y Axis