You will find BMW GSA panniers for sale regularly on this site. Other brands are available which the users will defend vociferously as THE ONLY ones to have bar none. Anyone who says differently is to be pilloried and subject to much abuse as is the custom in this site.
Yes you will have to change the footrest hangers and additionally source the pannier mounting brackets - commonly referred to here as ‘scaffolding’.
If you join the site and pay the subscription you will be able to access the for sale section where I believe right now someone is selling everything thing you need to fit GSA panniers to a GS.
Alan R