gcc compiler is already there, how to make point cc=gcc to proceed?

It shouldn't be necessary to do that, and I think the advice offered by roboticus is sound.

However, to answer your question:

cpanm Proc::ProcessTable --configure-args="cc=gcc"
[download]

or, if gcc location is not in $PATH environment variable:

cpanm Proc::ProcessTable --configure-args="cc=/full/path/to/gcc"
[download]

Cheers,
Rob