The MGallery by Sofitel branding has been used by the old Mercure Cheltenham since it rebranded as a MGallery earlier this month.
The Sofitel.com website now has a new tab on the home page "Sofitel Universe" which lists Legend, SO, Sofitel and MGallery as all being part of the overarching Sofitel brand. MGallery hotels however are currently not showing up as Sofitel destinations on the reservations part of the sofitel.com website. The sofitel.com website has had a minor makeover in terms of hotel destination listings.
If you link to MGallery from the sofitel.com website it simple redirects you to the old MGallery.com website which is still branded as MGallery by Accor.
Looks likely MGallery will increasingly migrate into the Sofitel website over the weeks ahead which unfortunately will probably mean more website downtime during the process.