I don't understand. Gabriel felt obligated to face the threat represented by the Forgotten One (given Laura's words about how it would basically wipe out everything), and the only way he could survive "other side" was by consuming Laura's blood. At that point, he was made aware that the blood would kill him if he didn't take it all, so he was forced to do just that and kill Laura in the process. Is that not "desperate" enough for you?
Not to mention he lost his wife and he feels like he fucked up the world, so he probably doesn't care to think twice about these sort of things anymore.
"I AM DARKNESS, I AM SHADOW."
Gabe isn't here right now, but if you'd like to leave a message his buddy Eu Sunt Dracul will be happy to get back to you.
It's kinda just odd that something as important as Gabriel turning into a vampire is relegated to DLC in the first place.
I completely agree with this, something this major should have been explained in the retail disc.