In the Prerequisites for GCC page, it states that a ISO C90 compiler is "necessary for building GCC". Does that mean I need a compiler to run another compiler(GCC) or is it only required if I want to compile GCC?

It is only required if you want to compile GCC. You do not need it if you just want to run GCC.

See GCC binaries for versions that you do not need compilation. In fact, under most normal conditions, there really isn't a point to building it from source anyway, since most packages on which GCC would be useful already come with it [or it is very simple to obtain it, such as through Cygwin].