How do I get llvm-gcc to point to gcc instead of clang? When I ask which version of gcc I have, I get the following:

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 9.1.0 (clang-902.0.39.2)
Target: x86_64-apple-darwin17.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin


When I try to get information about gcc: 
ls -la /usr/bin |grep gcc

I get:

-rwxr-xr-x     1 root   wheel     18288 May 29  2018 gcc
-rwxr-xr-x     1 root   wheel      2043 Sep 26  2016 gccmakedep
lrwxr-xr-x     1 root   wheel         5 Mar 28  2018 llvm-gcc -&gt; clang


I am using MacOS 10.13.5, and the reason that I want to use gcc instead of clang is because I have code I want to run in MATLAB that is in C, but I can't compile the code with clang, because fopenmp is unsupported.

When I try to compile just the matlab part of the program so that I can run it with a matlab interface:

make matlab

I get the following error:

gcc -std=c99 -pedantic -Wall -O3 -fopenmp -DSSHT_VERSION=\"1.2b1\"...


clang: error: unsupported option '-fopenmp'

I used the link: Installed gcc with Homebrew, now how to use that gcc instead of clang?

export PATH=/usr/bin
export CXX=/usr/local/Cellar/gcc/8.2.0/bin/g++-8
export CC=/usr/local/Cellar/gcc/8.2.0/bin/gcc-8


to my code.
Apple and xcode do not provide gcc. Any file called gcc us a link to clang

You need to install gcc from a third party and also check the version has mp enabled. 

The easiest way is to install gcc using a package manager like macports, homebrew or fink.