It looks like the GPS Map 298 has a standard BNC (BayoNet Connector) quarter turn connector for it's external antenna
You don't say which Furuno antenna you intend to use but the marine types tend to be have TNC connectors with a screw thread, Also the Furuno GPA-017 for example requires power to be supplied to it and as far as I can see GPS Map 298 manual does not mention supplying power to an external antenna