The master branch uses images to display emojis and therefore has a consistent display across all operating systems. The downside, however, is that there is no support for latest Unicode 10+ emojis – and will never be!
If you want to use the latest Unicode emojis displayed in the operating systems default emoji font please use the pre-release version v0.6.0 from the gemoji-4.0-native-emojis-no-images branch. It uses the gemoji gem and displays emojis as native Unicode emojis.