Faster's advice is sound on this. Much better to get the lead for the tack
line forward of the headstay.
[Here I'm assuming that by gennaker you are referring to a sail that is akin to an asymmetric cruising
spinnaker. If you are actually referring to a gennaker with a taut
line or wire in the luff, or one that is on a
furler, then the advice below does not apply.]
Folks that are using a padeye on the foredeck (aft of the headstay), as an attachment point for a block through which they run their tack
line, are usually doing so because the padeye was there in the first place as an attachment point for a conventional
spinnaker pole downhaul. If you don't have the padeye, and don't plan to fly a conventional (symmetric)
spinnaker, you don't need the padeye.
Also, the piece of equipment that you refer to as a parrel is not usually necessary if you have a true asymmetric
spinnaker/gennaker. Sometimes it's helpful to have one, but they are more often used when someone is trying to fly a symmetric
spinnaker like an asym -- i.e. without a pole.