By Homebrew policy, anything requiring a C/C++ compiler but doesn't compile with clang is out. So gcc not working shouldn't disrupt the overall Homebrew ecosystem too much. The biggest problem is gfortran, which is what the great majority of gcc dependents use it for (just `rg 'depends_on "gcc"'` in homebrew-core and all the "# for gfortran" comments should make this obvious).
Btw, here's the list turned up by a dependent search in core:
$ brew uses --recursive gcc
abyss dynare itk numpy qrupdate
adios2 eccodes itpp numpy@1.16 r
apache-arrow ensmallen jags nwchem ratfor
apache-arrow-glib fastme joplin octave rawtoaces
arcade-learning-environment fftw json-fortran ola raxml-ng
armadillo field3d kahip ompl reprepro
arpack flann kallisto open-mpi root
arss fobis kim-api openblas scalapack
astrometry-net gdal lammps opencoarrays scipy
asymptote gdcm lapack opencv shogun
aubio gmic libbi opencv@2 simple-tiles
augustus gmsh libmatio opencv@3 siril
biosig gmt libxc openfast skymaker
boost-mpi gmt@5 liquid-dsp openkim-models sratoolkit
bsponmpi gnuradio luaradio osi suite-sparse
caffe gr-osmosdr mapnik osm2pgrouting sundials
calceph grace mapserver otf2 superlu
cdo graph-tool minimodem packmol synfig
ceres-solver gromacs mlpack pcl urh
cgns gwyddion mpdviz pdal veclibfort
clp hackrf mpi4py petsc vips
coinutils hdf5 mpich petsc-complex virtualpg
cp2k hdf5-mpi mvtools pgplot visp
cpl hdf5@1.10 ncmpcpp pgrouting vtk
csound hdf5@1.8 nco plplot waon
dartsim hypre ncview pnetcdf wxpython
datetime-fortran imake nest points2grid zita-convolver
dlib imgproxy netcdf postgis
dungeon inspectrum networkit proteinortho
dvc ipopt ngspice qd
Edit: Forgot to --include-build. That would add 4:
boost-python3 btfs libtorrent-rasterbar openimageio