Join two group_concat result
You can reverse the order of the two functions, so the concatenation of group_name and id is done first - using either CONCAT() or CONCAT_WS() - and then the group concatenation:

GROUP_CONCAT( CONCAT_WS(':', rtg.group_name, rtg.id) 
              ORDER BY rtg.id
              SEPARATOR ','                      -- the comma ',' is the default
            ) AS groupList