I feel foolish because I figured it out right after posting, but I figured I'd share because it isn't intuitive and hopefully this will save someone some time.

The package you want to install is not gcc. You want to install gcc-toolset-9. You will notice after installation that you will still get 8.x if you do a gcc --version. gcc-toolset-9 installs to /opt. The idea is to give you a separate development environment. See this documentation. After installation you can run the updated gcc with /opt/rh/gcc-toolset-9/<username>/bin/gcc.

As @Stephen Kitt pointed out, you can actually get a shell with the updated toolset by running scl enable gcc-toolset-9 bash. Running this command will open a new bash session with the environment variables appropriately updated.

You can also run gcc v.9 directly with scl enable gcc-toolset-9 gcc <your_gcc_args>. See this documentation for details.

yum manual:

deplist

Produces a list of all dependencies and what packages provide those dependencies for the given packages.

Try somenthing like:

$ yum deplist gcc

Output:

package: gcc.x86_64 4.8.5-4.el7
  dependency: /bin/sh
   provider: bash.x86_64 4.2.46-20.el7_2
  dependency: /sbin/install-info
   provider: info.x86_64 5.1-4.el7
  dependency: binutils >= 2.20.51.0.2-12
   provider: binutils.x86_64 2.23.52.0.1-55.el7
  dependency: cpp = 4.8.5-4.el7
   provider: cpp.x86_64 4.8.5-4.el7
  dependency: glibc-devel >= 2.2.90-12
   provider: glibc-devel.x86_64 2.17-106.el7_2.8
   provider: glibc-devel.i686 2.17-106.el7_2.8
  dependency: ld-linux-x86-64.so.2()(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: ld-linux-x86-64.so.2(GLIBC_2.3)(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: libc.so.6(GLIBC_2.14)(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: libdl.so.2()(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: libdl.so.2(GLIBC_2.2.5)(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: libgcc >= 4.8.5-4.el7
   provider: libgcc.x86_64 4.8.5-4.el7
   provider: libgcc.i686 4.8.5-4.el7
  dependency: libgcc_s.so.1()(64bit)
   provider: libgcc.x86_64 4.8.5-4.el7
  dependency: libgmp.so.10()(64bit)
   provider: gmp.x86_64 1:6.0.0-12.el7_1
  dependency: libgomp = 4.8.5-4.el7
   provider: libgomp.x86_64 4.8.5-4.el7
   provider: libgomp.i686 4.8.5-4.el7
  dependency: libgomp.so.1()(64bit)
   provider: libgomp.x86_64 4.8.5-4.el7
  dependency: libm.so.6()(64bit)
   provider: glibc.x86_64 2.17-106.el7_2.8
  dependency: libmpc.so.3()(64bit)
   provider: libmpc.x86_64 1.0.1-3.el7
  dependency: libmpfr.so.4()(64bit)
   provider: mpfr.x86_64 3.1.1-4.el7
  dependency: libz.so.1()(64bit)
   provider: zlib.x86_64 1.2.7-15.el7
  dependency: rtld(GNU_HASH)
   provider: glibc.x86_64 2.17-106.el7_2.8
   provider: glibc.i686 2.17-106.el7_2.8

Hope It Helps!