glxgears is a small and simple MESA OpenGL demonstration program that comes with GNU/Linux distributions. It's technically a part of mesa-demos but it ships with the glx-utils package and is thus installed as a default (unlike other mesa-demo programs like glxheads, shape, offset, etc).
It's not a benchmark
A lot of pages on the Internet refer to glxgears as a benchmarking tool. It's not. It's just a very simple OpenGL demo.
Run some actual games like SuperTuxKart with a frame and/or frame-time heads-up display overlay for real-world graphics performance or use glmark2 or vkmark2 for a bit more meaningful synthetic benchmarks.
glxgears can be run in
-stereo mode to test VR headsets. This option will not work if you do not have a VR (stereo) display.
-display to choose which monitor it will run on,
-fullscreen to run it fullscreen and
-geometry set by
WxH+X+Y to launch it at a given resolution (WxH) placed a specific place (X+Y). There is also a
-fullscreen option is a bit buggy. It will actually launch glxgears fullscreen over two monitors on a triple monitor setup.
Mesa's HUD options for getting a "heads up" display showing FPS and other data on AMD GPUs can be used with
The above image with the small HUD graphs above
glxgears shows what happens when you launch it with:
GALLIUM_HUD=".w250.h180.c70fps:70,.w250.h180.c80temperature:80" glxgears -geometry 1800x1000
There are a lot more HUD options available. Run
GALLIUM_HUD="help" glxgears to see them all.