Recognize invitations from Gmail
Outlook.com, Outlook desktop, and Outlook for Windows 10 all recognize calendar invitations from Gmail and give options for replying (Accept, Tentative, Decline). Outlook Android recognizes invitations from other sources, but didn't give the response options to emails from Gmail.
Tania Kodz commented
I am only able to view Gmail calendar invites and am not able to accept them without having a Gmail account.