It's one of the most widely used compilers of C, C++, Fortran, and several other languages. Much of the software in the Open Source world, from the Linux kernel to the shells, text editors, tools, games, etc. was written to be built with GCC. Chances are if you've used a computer, even a Windows machine, and certainly if you've used a Mac, or an Android phone, or countless other devices, you've used software that's been compiled with GCC.
I've personally used GCC quite a bit in my own software engineering career.
Here's to an unsung pillar of today's computing world.