git add submodule

git submodule update --init --recursive

git submodule update --recursive --remote

git clone --recurse-submodules

git submodule update --init --recursive

git submodule update --remote <path to the submodule>

git submodule add path/to/submodule

git checkout --recurse-submodule

# Be careful it will delete all untracked files, reset all stagated files
# and will remove all submodule folders
git reset --hard
git checkout gh-pages
git clean -xffd
# Optional restoration of submodules content
git submodule update --init --recursive