Supported OData Query Options
OData supports query options like $filter=cityfrom eq 'SINGAPORE' or $select=...
(A full list can be found [here|http://www.odata.org/media/16352/%5Bms-odata%5D.pdf])
1) Which options are supported by Gateway?
2) What is the "Gateway Solution" to filter the result of the GetList/Query command for a specific entity?
(e.g. return all flights with city of departure equal to 'Singapore')
Supported options are listed in [Note 1574568 - SAP NetWeaver Gateway 2.0 - Known Constraints|https://service.sap.com/sap/support/notes/1574568]. There are currently some limitations for query options like $filter.
But it can handle what you describe (e.g. filtering flights by depart city). It would be just as you had it. For example, .../FlightService/FlightCollection?$filter=cityfrom eq 'SINGAPORE'&...