I plan to fine my beer in the keg with gelatine. I read that there will be some trub that sinks to the bottom and will come out in the first few pints. Is this "trubbie" beer drinkable/representative of the rest of the keg?
It's "drinkable" but you're drinking the stuff you went through effort to remove with gelatin. I'd recommend pouring it out. It won't hurt you although some folks say drinking lots of yeast makes them a bit..eh..gassy.
The beer should clear up after the first few pints are poured off (2-4) and get clearer for a few days until it's nearly perfectly clear.
I tend to find that with care, you don't get this crud coming out as long as your tap is an inch off the bottom. There might be some in the tap the first time you pour any but I don't have issues otherwise. You don't want to be moving the keg!
It should be obvious if you are getting it, as the beer won't be clear. Maybe you drink that part yourself to get the keg 'ready for sharing' until it runs clear ;)