How can you find out which version of GCC/GNU compiler your are using?

On the command line, enter

gcc --version

What Moschops said, but you can have multiple versions of gcc installed on your system. The command shown will be the default version installed on your system. I have both 4.4.7 (default) as well as 4.7.2 installed. At Fermi National Laboratory, they are running 4.9.1 by default as I understand (my wife works in the computing division there). That is the latest and greatest with the most C++14 support available, given the standard is still evolving.