SLMG is not used in Part SFCL and so there is no such thing as an SLMG rating to go on an SPL. What you get in an SPL is a a Sailplane/powered sailplane rating. You can use this to fly what was an SLMG if you also have 'Self-launch' as one of the launch methods on the 'BGA SPL Logbook Signature Record' page in the licence.