Sample SOQL to get Permission Set Names assigned to the Permission Set Group:

SELECT PermissionSetGroup.DeveloperName, PermissionSet.Name 
FROM PermissionSetGroupComponent

Sample SOQL to get Users assigned to the Permission Set Group:

SELECT Assignee.Name, PermissionSetGroup.DeveloperName 
FROM PermissionSetAssignment 
WHERE PermissionSetGroup.DeveloperName = 'All'