GCC is the compiler that creates linux programs.
Sometimes software is released for older versions of GCC so that they can run on older Linux operating systems which do not support the newer GCC version. GCC is a compiler so you might be wondering why supporting a newer version of a compiler is important. It's not the compiler
per se that needs to be new enough, it's the C standard library (libc) bundled with it that needs to be new enough to be understood by the compiled-with-new-GCC program.
If you try to run programs compiled with newer GCC on systems where only older GCC version is available, the program will fail to run because libc library is too new for the program to understand it.
In Armory's case, two versions of the wallet are available which use GCC 4.7 and GCC 5.3 respectively. Certain important operating systems like CentOS 7 ship with GCC 4.8, so the GCC 4.7 build is the only one that works on it.
And this is why if you compile Armory on Ubuntu 20.04 and package it into an installer, it will only run on Ubuntu 20.04+, it will
not run on 18.04, 16.04 or anything else under 20.04 because the newer GCC and libc on 20.04 is not found in older distros.
Where are you seeing these to versions to download?
What exactly are you attempting to do here? Just download and run Ubuntu with Armory+Bitcoin Core?
Most likely from the Armory download page
https://www.bitcoinarmory.com/download/
