Xorg – change display resolution on AMD GPU, Debian

When I tried to create a new mode in Xorg (via xrandr --newmode), I get the following error:
Failed to get size of gamma for output default
The solution turned out to be simple: I just had to install the firmware-linux-nonfree package and reboot the PC.
So, here is the complete resolution change process:

1. cvt <width> <height> <refresh rate>, output may be something like this:
   # 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz Modeline "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
2. xrandr --newmode "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
3. xrandr --addmode <OUTPUT> 1920x1080_60.00, OUTPUT can be extracted from xrandr output. It can be something like DVI-0, HDMI-0 etc
4. To apply new mode that you created execute xrandr --output <OUTPUT> --mode 1920x1080_60.00

After that, the screen resolution will change to the one you specified.