Google has been working on the emoji reactions feature in Gmail (Android & iOS) which was reported by Steve Moser (for iOS) and then by TheSpAndroid (for Android). Initially we got hint of the feature from the strings found in the app, but now I have an exclusive looks at how it is going to work. I managed to enable it my Gmail (Android) and it is working as expected. Also got a video for you all to see it in action.

How it works - 

When rolled out to users, you will see a smiley 🙂 icon to the right of the email (just before the three dot menu). Clicking on it will show you onboarding tooltip with the text - Respond quickly and add personality - Gmail users see your reaction in the original message. Others will receive it as a reply email. You will see an emoji reaction bar, just like Google Messages, just below the tooltip.

Here you can react with 5 pre-defined emojis and there is a + button with which you can open all emojis and react with any of them. There are 3 places from where you can react on an email - the emoji button just before the 3 dot menu, the "Add reaction" option in the 3 dot menu itself and there is a emoji button on the right side of reply, reply all and forward button in bottom on the email screen. Also it may be possible to react just by tapping the emojis if someone already reacted on the email.

When you react on an email through Gmail on Android (or iOS), if the other person also uses Gmail on Android/iOS, he/she will see the reactions on the bottom of the email. But if the  other person doesn't use official Gmail app, he/she will see the reaction as a separate email with - " (emoji) (name) reacted via Gmail on (date)", e.g. 🔥 assembleDebug reacted via Gmail on 29 September 10:00am". There is no idea about the emoji support on the Web version of Gmail but I am sure it will soon because Google doesn't want their user's web version Gmail spammed with separate emails of emoji reactions.

There are some limitations which we previously reported when we found out about this feature. Here a they -
• If you receive email via BCC, you can't react with an emoji.
• You can't react with an emoji to an encrypted message.
• You can't react with an emoji to a large group such as emails sent to a large number of people by companies 
• You can't react with an emoji more than (number) times.
• You cannot add more than 20 emoji reactions to a message.
• There can be only 50 unique reactions to an email.

And here is a full video showing the emoji reaction process in action - 

The rollout of this feature will begin soon in batches. Probably will be rolled out in October at any time. For more Google apps feature leaks and flags to enable them (Root), you can follow me on telegram on Gapps Flags & Leaks channel. ;)