Use `gsed` for compatibility of `sed` on OSX

Command line tools for those coming from a unix backgroud often have issues ensuring things just work

I've run into multiple issues of such kind using sed on OSX. That's which I discovered gnu-sed. As I understand it, it the version of sed that runs in OSX and is compatible with the unix version commands.

Installation is simple:

brew install gnu-sed

It gets installed at /usr/local/bin/gsed

Fire up gsed as use it as you would normally do in a unix environment. Everything seems to work as expected for now!