sortExpression 
  Instructions on how to sort the search results. If no sort expression is included, the request returns results in random order. A sort expression consists of the following elements:
- Operand -- Name of a game session attribute. Valid values are - gameSessionName,- gameSessionId,- gameSessionProperties,- maximumSessions,- creationTimeMillis,- playerSessionCount,- hasAvailablePlayerSessions.
- Order -- Valid sort orders are - ASC(ascending) and- DESC(descending).
For example, this sort expression returns the oldest active sessions first: "SortExpression": "creationTimeMillis ASC". Results with a null value for the sort operand are returned at the end of the list.