The other part of the update gives users a way to respond to invitations from Google Calendar or Microsoft Exchange, quickly and painlessly. The Instant RSVP feature allows you to see your schedule, check for conflicts, and respond to the invitation all with one tap.
The update could hit the Google Play Store as soon as today. Open the Play Store app, tap on the hamburger menu in the upper left corner, click on My apps & games, and look to see if the update has arrived. If not, it should be there soon.
Gmail for Android adds RTF support and quick response to RSVP invites
1. RTF support comes to Gmail for Android
2. RSVP with one tap after installing this update
source: Gmail (Android) via TheVerge, TechCrunch