gs.action.getGlideURI().get('parameter_name');
Another longer alternative:
gs.action.getGlideURI().getMap().get('sysparm_query');
var transaction = GlideTransaction.get();
var targetRecord = transaction.getRequestParameter('sysparm_record_target');
Used in ApproverUtils SI: nav_to.do?uri=sys_script_include.do?sys_id=4b28d4fcc78533003725756d5ec2600c
This does not work in before query business rules. I had some success with
var targetRecord = RP.getParameterValue('sysparm_record_target')
But this creates an error entry in the syslog every time it runs...