Ray's advice is right: Connect the RS-232 output to a computer. Tx on the GNS to Rx on the serial adapter (pin 2), signal ground on the GNS to ground on the serial adapter (pin 5). Configure the software in the way you expect the GNS to output (baud, data, parity, stop bits), and verify data shows up in the software's terminal screen. If you are expecting NMEA, you'll see text sentences that kind of look like this:
$GPGSV,2,1,08,02,74,042,45,04,18,190,36,07,67,279,42,12,29,323,36*77
$GPGSV,2,2,08,15,30,050,47,19,09,158,,26,12,281,40,27,38,173,41*7B
For software, I don't have much of a recommendation for Mac, but it's simple software--they all should work. Don't pay money for one.
Some "cheaper" USB to serial adapters can be (or at least used to be, in my day) flaky or simply not work. You should be fine with Belkin or some other big name. To test yours out before connecting to the GNS, wire pin 2 to pin 3, open up your software, and start typing. If you see what you type, it's working. When doing this professionally, I would always use ancient computers that have "real" serial RS-232 ports sticking out of them, rather than the adapters.
Good luck!