How do I accept invitations in the Gmail app?

Google Calendar's invitations are emails with a few links to accept or decline the invitation, Apple's iCloud have something silimar, but Outlook.com / hotmail calendar only have a invite.ics and that's it.

But when you have received an email in Gmail for an invitation, it's actually already added on your calendar and visible in DigiCal and any other app on an Android device. This is because Gmail understands how to add .ics from invitations directly to your calendar, you don't have to do it manually (at least for Google Calendar, Outlook.com, Exchange and iCloud, I haven't tested anything else myself).