I do not believe DM5 has a version fir Chromebook but other logbook apps, such as Subsurface and Dive Log have android apps that should work. Besides, don't even try using DM5 as it is a worthless piece of software. I don't think it will download from a Gekko (I never could get it too, anyway) but I do know that Subsurface will. It's what I've been using for a while now. I no longer use the Gekko as it crapped out a few years ago but my wife and I still use Suuntos.
As for the cable, a new factory cable costs around $80.00 or so. Maybe someone here on SB will have one they will sell you or check out ebay, amazon, or some other online sellers for a used one.