remove_gem.sh

# Note: I'm assuming GNU sed, this would need tweaking to run on OSX
# which I'm not inclined to research/test right now.

cat > Gemfile.example <<EOF
gem "awesome"
gem "lame"
EOF

echo -e "\nBefore:"
cat Gemfile.example


sed -i '/gem "lame"/d' Gemfile.example


echo -e "\nAfter"
cat Gemfile.example


# Or if you prefer generalized and reusable
remove_gem() {
  sed -i -r "/gem\s+('|\")$1\1/d" "${2:-Gemfile}"
}

remove_gem lame Gemfile.example

I believe the OS X version requires an extension parameter supplied to -i (.bak or similar)