As you can imagine, Android works really different from iOS in many things. Keyboard is one of that. Both platforms let users to install third party keyboards (as this emoji one) but the feature works different.
Keep in mind also that there are more than 10,000 android devices with different settings, so it's practically impossible to make a regular keyboard for all. That's why we integrate the emojis on the Google Keyboard.
iOS (iPhone and iPad): This keyboards works as the famous "copy & paste" function. So when you tap an emoji from the keyboard, it's copy on your clipboard and you can paste it on the app that you are using. (Facebook comments, Mails, instagram messages...)
Android works different. There is no copy & paste function. When you tap an emoji, it creates automatically an MMS to send in any app that includes a message feature: facebook messenger, whatsapp, hangouts... Normally if you are in one message app, it will create a new message on that same you are. The problem is on the app that has no messenger, like Gmail. When you tap an emoji on the keyboard in Gmail, the system can't create a new MMS (because there is no messenger) so nothing happens.
¿Solution? The one that we always recommend: Use the emojis from your regular Google Keyboard. They will always work in any app and you don't need to install the custom keyboard.