Hey,
If it has two clews and no tack then it's a regular spinnaker and not a gennaker.
You can still try flying it like a gennaker by tacking one of the clews to the front of the boat. Most people us an adjustable tack line so the tack can be raised or lowered depending on point of sail, wind speed, etc.
Do you have the sheets, blocks, tack line, and everything else needed? I would take some time to make sure you have all the gear and know how to run all the lines. Then pick a day with light wind, say under 10 kts and give it a try. Make sure you have plenty of sea room.
There are a number of good youtube videos out there that show how to rig, launch, fly and douse a spinnaker with a dousing sock.
Good luck,
Barry