To uninstall it open a powershell as administrator and uninstall it with the following command:
Get-AppxPackage Microsoft.XboxGamingOverlay | Remove-AppxPackage
However this left me with a nagging popup upon game start:
you'll need a new app to open this ms-gamingoverlay-link
To resolve this you can also use the powershell to edit the registry like this:
reg add HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\GameDVR /f /t REG_DWORD /v "AppCaptureEnabled" /d 0
reg add HKEY_CURRENT_USER\System\GameConfigStore /f /t REG_DWORD /v "GameDVR_Enabled" /d 0
This disables the game recorder software which seems to still think that the gaming overlay is still present.
Hope this helps some people as it was not obvious at all for me how to get rid of the gamebar/overlay without getting the nagging popups when still trying to game.