opening up gmail in Edge Beta...

You're accessing the mobile/minimal version of Gmail.
try pasting this into your address bar see if it loads normally:

https://mail.google.com/mail/u/0/#inbox

that should load the desktop/normal version of gmail for you.

you can also edit your gmail bookmark and set it only to "mail.google.com"

the browser should take care of the rest based on your user-agent (mobile/computer)

if the above URL fails to load the desktop version of gmail for you and the problem persists, try:

1. disabling your extensions temporarily, in case they are causing user-agent incompatibility.

2. clearing your browser data: edge://settings/clearBrowserData

3. use newer version of Edge insider browser such as Dev and Canary to see if it keeps happening again.

hope that helps