In that case, the issue is Gateway side. You miss some setting.
Go to transaction /IWFND/ERROR_LOG. You can check call stack, set a break point and debug.
Other possibility is that you set wrong value for system alias.
Go to transaction /IWFND/MAINT_SERVICE. Select the service ZLEAVEREQUEST, and call Gateway Client.
Test oData calls. Probably you will get an error. Double check the system alias setting.
/sap/opu/odata/GBHCM/LEAVEREQUEST;v=2/$metadata
/sap/opu/odata/GBHCM/LEAVEREQUEST;v=2/?$format=xml