This is a .gold file editing package. you can see about gold here.
From MELPA, you can install like this:
M-x package-install gold-mode RET
If you installed from MELPA, this package may change to gold-mode automatically when you opened .gold extension file. So you don't need to write below codes.
Below part is for manual(git or el-get) installation user:
(add-to-list 'load-path "path/to/gold-mode-directory")
(autoload 'gold-mode "gold-mode")
(add-to-list 'auto-mode-alist '("\\.gold$" . gold-mode))