Better still, I'll recommend @kana's vim-textobj-entire[1]. This adds 'e' as an 'everything' text object, with 'ae' (around everything) and 'ie' (inside everything) bindings by default. Thus 'vie' would select everything in the buffer in visual mode. Likewise for 'Vie' (Visual line select) or 'cie' (change everything).

[1] https://github.com/kana/vim-textobj-entire

Also you don't need any visual mode if you don't need visual feedback. Yanking everything:

    gg"zyG

Or

    :%yz

Good call! But you'll need a space:

    :%y z

If I can recall the file name, :r <file name> might be a better choice than gg...y