Easy as Homebrew itself is:

brew install git-flow-avh

brew install git-flow-avh --HEAD

As per version 1.7.0 there is no need for any post installation setup. If you upgraded from an earlier version you can remove export FLAGS_GETOPT_CMD="$(brew --prefix gnu-getopt)/bin/getopt" from the file ~/.gitflow_export.

For git-flow prior to 1.7.0

Create a ~/.gitflow_export with the content export FLAGS_GETOPT_CMD="$(brew --prefix gnu-getopt)/bin/getopt".

Even using wget its a one line effort.

wget --no-check-certificate -q -O - https://github.com/petervanderdoes/gitflow-avh/raw/develop/contrib/gitflow-installer.sh install stable| sudo bash

wget --no-check-certificate -q -O - https://github.com/petervanderdoes/gitflow-avh/raw/develop/contrib/gitflow-installer.sh install develop| sudo bash

wget: command not found? curl is only two. (Note that URL is where the above URL currently redirects.)

curl https://raw.githubusercontent.com/petervanderdoes/gitflow-avh/develop/contrib/gitflow-installer.sh > gitflow-installer.sh
chmod a+x gitflow-installer.sh

curl -sL https://github.com/petervanderdoes/gitflow-avh/raw/develop/contrib/gitflow-installer.sh | sudo bash -s install stable

curl -sL https://github.com/petervanderdoes/gitflow-avh/raw/develop/contrib/gitflow-installer.sh | sudo bash -s install develop

Install GNU getopt via Homebrew:

brew install gnu-getopt

Create a ~/.gitflow_export with the content export FLAGS_GETOPT_CMD="$(brew --prefix gnu-getopt)/bin/getopt".

For git-flow versions prior to 1.4.0-dev.28
Create a ~/.gitflow_export with the content alias getopt="$(brew --prefix gnu-getopt)/bin/getopt".

If you have installed GNU getopt through other means than Homebrew, substitute $(brew --prefix gnu-getopt)/bin/getopt with the location of the GNU getopt file.