Believe it or not, there is no bilge pump installed on production Gemini's.
The shower sump pump (250gph) has a T valve in a locker on the port hull and hoses that extend to each hull; you can uncoil the hose and use it to vacuum out any loose water that makes it into our 4 inch deep bilges. The pump drains via a t valve in the head's sink drain.
Fortunately Gemini's are dry boats.
I keep promising myself that some day I'll install a real bilge pump but running a hose to topside is going to be a real problem. For now I have a 1500 gph Rule wired to a 12v plug that has a hose long enough to reach my cockpit as a just in case (there is a 4 ft head for those that are interested). I also use this for anchor washdown, just throw the pump overboard, plug it into a 12v plug in my anchor locker and use the hose end to spray away mud. It's not fresh water but it's better than nothing.
BTW the Ocean Cat A certified version of the Gemini has a manual pump in the lazerette, switchable to either hull and a 1500 gph electric pump in each hull. They can't retrofit the installation once the hull liner is bonded in which is why I'm having a hard time figuring how I'll route the hoses.
I may just go with a check valve and a hose to the centerboard slots, but if the check valve fails my boat would sink about 2 feet.