After resisting for a bit, I decided to try the plugin manager vim-plug.
It seemed to work well when installing the GhostText plugin, except that I had to do the installation in a sudo nvim session, ...
I have installed gruvbox using vim-plug, then I installed Airline and set the theme following the recommendations. But every time I close Vim and open it, the theme for Airline is reset to default and ...
My process so far:
Brew unlink vim
Brew install macvim
Brew link macvim
Installing YCM with vim-plug.
Running ./install.py from YCM folder.
cmake -G "Unix Makefiles" .