Here on the dowloads doc (https://phalconphp.com/en/download) :

git clone --depth=1 git://github.com/phalcon/cphalcon.git
cd cphalcon/build
sudo ./install

Just add a specific doc for gentoo because it's not the same script :

git clone --depth=1 git://github.com/phalcon/cphalcon.git
cd cphalcon/build
./install-gentoo