It's most likely that your Gengar is from another country. Unless the Gengar has perfect IVs or somesuch I'd suggest just breeding another male Gastly to preserve any egg moves and keep trying with that one holding the power item.
80*1.5*1.5/2 vs 100*1*1/1. If you're good at your mental math, you'll note that Earthquake is a small margin better in this situation. Please note, however, that if you were using Aqua Tail it would have 1.25 more BP than Earthquake at the cost of 10 accuracy.
In Platinum if you pressed Y and clicked and dragged from a pokemon in the corner of your box to the opposite corner it would highlight every pokemon in the box, meaning you could transfer every single one in a matter of seconds. Heart Gold's system is workable, and I find it to be quite convenient, but Platinum's system is still faster.
The Move Item feature is also lacking, you can not move items from one box to another without first putting the item onto a pokemon in your party, changing boxes, and then putting the item from your party pokemon onto your boxed pokemon. Very frustrating.
It does not matter what patch of grass you're in, I found two in the exact same patch of grass (One of the ones in the route east of Ecruteak town) within the span of thirty seconds. It's simply based on luck.
Breeding, battling, and dastardly mentions of AR codes.
The EV boosting effect still applies, though, there is no Vs. Seeker, so no EV trainers, excepting the fisherman you can rebattle once every thursday with six Magikarps... Fighting wild pokemon is easier.