You can use modern plugin managers to download the GN repo and manage the vim plugin:
Example config for vim-plug:
Plug 'https://gn.googlesource.com/gn', { 'rtp': 'tools/gn/misc/vim' }
Or, for Vundle users:
Plugin 'https://gn.googlesource.com/gn', { 'rtp': 'tools/gn/misc/vim' }
If you don't use a plugin manager or would prefer to manage the GN repo yourself, you can add this explicitly to rtp in your .vimrc:
set runtimepath+=/path/to/src/tools/gn/misc/vim " ... filetype plugin indent on " or a similar command to turn on filetypes in vim