tinyE: I was never a huge fan of the first one which I realize makes me the world's biggest dickhead but hey, gotta keep people honest. :P
If you're the biggest dickhead in the world for not being a huge fan of the first Blood Omen, that makes two of us. I think the first game is largely overrated; it's nothing more than a Diablo/Zelda clone with your standard vampire revenge story.
I'd be happy to have it on GOG, for completion's sake, but I honestly don't care all that much that they can't sell it. As far as I'm concerned, the Amy Hennig titles (Soul Reaver, Soul Reaver 2 and Defiance) are on GOG, and those are really the only Legacy of Kain titles you need.
If you think you're missing out on the story for not playing Blood Omen (you're really not, everything is explained better from Soul Reaver onwards), you might as well watch the cinematics on YouTube before playing any of the other games. Or get a system with access to PSN and buy Blood Omen there.