What kind of interface card do you have in the Sprint? You have to have one. It will be plugged into the motherboard of the Sprint.
I use a Gerber LMK card (Lettering Machine Kit). It is plugged into a slot in the motherboard and has a cable that plugs into the card and the other end is then connected to a COM port on my PC. If your are plugging it into the Flexi dongle, you are attempting to run it from an LPT printer port. Gerber doesn't support this kind of a hookup ... not sure about Flexi but I doubt it. COM ports are male and come in either 9 pin or 25 pin. You should be plugging your cable into a connector on the back of your computer that looks like this (black dots are male pins):
If you are attaching to any port on the Sprint which is on the back of the console, you are not attaching to a valid port.
GDS Mode is for bringing a digitized file in from an obsolete Gerber Tablet System. When I run my Sprint, I press the top left function key which is labelled "Online". The next function key to the right is labelled "Offline".