Basically your Fiori app consumes the same OData service as your oData API , and at Gateway you cannot tell the consumer.
Of course for your Fiori app , you can put some indicator in the HTTP call(Header) or similar approach to let Gateway knows that those requests are sent from your Fiori App. but cusumer of OData API can do the same to trick your backend logic.
If you have strong business reason to differentiate the consumer, please consider to have a separate oData API( new OData service and reuse your backend logic as much as possible).
solved How to differentiate whether a call to SAP is from OData API or Fiori app