1. You should be able to. But there is no reason to, just upgrade the old version to a new version.
2. Assuming that you installed gcc as a rpm (The version that comes with redhat is installed by rpm), simply "rpm -e gcc" at the command line. I don't suggest that you remove gcc though, as other programs depend on it. Try this:
3. Find an rpm version of gcc. Install it using "rpm -uvh gcc-version.rpm" This will upgrade your current version of gcc to the new version that you have downloaded.
To answer your question:
tar -xzvf fileName.tar.gz
./configure && make && make install
as root should do the trick
I hope this helps