gcc on macOS is running from /opt/local instead of /usr/bin - c++

Question

I was trying to upgrade my gcc version using brew and copying the new version file to usr/bin by using:
sudo ln -s /usr/local/Cellar/gcc/9.3.0/bin/c++-9 /usr/bin/c++
sudo ln -s /usr/local/Cellar/gcc/9.3.0/bin/g++-9 /usr/bin/g++
sudo ln -s /usr/local/Cellar/gcc/9.3.0/bin/gcc-9 /usr/bin/gcc
sudo ln -s /usr/local/Cellar/gcc/9.3.0/bin/gcc-9 /usr/bin/cc
then I add the path to .bash_profile as below
PATH="/usr/local/Cellar/gcc/9.3.0/bin:/usr/local/Cellar/gcc/9.3.0/lib:${PATH}"
export PATH
after all these done, I ran gcc -v, there is no change on version and I found the gcc it gave me was from /opt/local instead of usr/bin. Any idea about this situation?
The terminal below is the gcc -v which computer gives me.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/local/libexec/gcc/x86_64-apple-darwin16/5.4.0/lto-wrapper
Target: x86_64-apple-darwin16
Configured with: /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_gcc5/gcc5/work/gcc-5.4.0/configure --prefix=/opt/local --build=x86_64-apple-darwin16 --enable-languages=c,c++,objc,obj-c++,lto,fortran,java --libdir=/opt/local/lib/gcc5 --includedir=/opt/local/include/gcc5 --infodir=/opt/local/share/info --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-5 --with-local-prefix=/opt/local --with-system-zlib --disable-nls --program-suffix=-mp-5 --with-gxx-include-dir=/opt/local/include/gcc5/c++/ --with-gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --with-isl=/opt/local --enable-stage1-checking --disable-multilib --enable-lto --enable-libstdcxx-time --with-build-config=bootstrap-debug --with-as=/opt/local/bin/as --with-ld=/opt/local/bin/ld --with-ar=/opt/local/bin/ar --with-bugurl=https://trac.macports.org/newticket --with-pkgversion='MacPorts gcc5 5.4.0_0'
Thread model: posix
gcc version 5.4.0 (MacPorts gcc5 5.4.0_0)