for installing gcc, one of the prerequisites is ISO C90 compiler for bootstrapping gcc, as per this link http://gcc.gnu.org/install/prerequisites.html
I have searched web and could not find any such thing ' ISO C90 compiler' . what exactly the website is refering to?
That applies only if you're building gcc from source. On most systems, it's much easier to install a binary distribution, which doesn't require an existing C compiler (because whoever created the binary distribution used a C compiler to compile gcc for you, or got the binaries from someone else who did).
C90 is the 1990 version of the ISO C standard. Since gcc is implemented in C, you need an existing C compiler to build it. This can be an earlier version of gcc, for example. If you tell us what operating system you're using, we can give you a better idea where to find a C compiler you can install.
As for the "ISO C90" requirement, just about any C compiler you're likely to find will support ISO C90 at least well enough to bootstrap gcc. The phrase "ISO C90 compiler" is very nearly equivalent to "C compiler". (Only some very old C compilers will fail to support C90.)