Extracting emojis does not work as described in the Readme at all, neither on Mac OS El Capitan nor on Mac OS Sierra. The command gemoji extract public/images/emoji was introduced for 3.0.0.rc1, so the current description does not match the behavior of the version 2.1.0 that is installed currently using bundle install. Previously the gemoji gem apparently used rake emoji but this does not work either for the gem version 2.1.0. This leaves the user a bit confused 😕 .
gemoji extract public/images/emoji
The README for the master branch only applies to the edge version, of course, which is currently released as 3.0.0.rc1. Yes, previously there was a rake task, but it won't work on macOS Sierra due to changes to its Apple Color Emoji font. Can you try the gemoji extract command from 3.0.0 and tell us if it works?
I only have Mac OS El Capitan, but my colleagues who use Sierra reported they could not get it to work, probably because of the transition from 2.1.0 to 3.0.0. As a workaround it is possible to download 2.1.0 where the emojis are contained in the images/emoji/unicode directory. Is it legal to use them, BTW? Extracting emojis from a font and turning them into bitmapped PNG images could cause a copyright problem.
I can't give you any advice about the legality of your decisions. The latest version of gemoji doesn't ship with bitmaps anymore, just with a script to extract them if you desire to do so. What you do with that script is up to you. The script will only work on Sierra and later.