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.