Google says that an issue with adding Gmail accounts to Windows 10 Mobile's Outlook Mail client has now been fixed.
Windows 10 Mobile users in our forums and elsewhere started reporting this issue a few weeks ago. People who tried to add a new Gmail account in the Mail app got a "Your browser isn't supported" error message instead. Some people found workarounds to this issue, but just for emails and not for syncing contacts or calendar events.
Sorry for this problem everyone, this was not intentional and has now been fixed. If you are still having trouble adding your Google account, please let me know.
The rep did not offer a statement on what caused the problem in the first place. Let us know if you can add Gmail accounts to Mail in Windows 10 Mobile in the comments!