Zadruga are revenants. Revenants are the childe of ghouls that are born with vampiric blood. So they are "hereditary ghouls" in a way.

What does it mean they are born with vampiric blood, where is the blood coming from? I got a Zadruga with my ghoul MC and a Mortal. Is there a lorewise reason why you can't become a hunter if your a Zadruga and part of the Gantebein house. It doesn't makes sense for me but maybe you can explain it to me?

1) personally I think revenants/zadruga should be able to become hunters (i know some people will disagree, but that's my take). Revenants are just as capable (quite literally more in some cases) of hunting vampires as any other hunter. However, if i had to guess, you can't do that in the game because various groups (unsure about the Gantebein, i don't know much about their lore) wouldn't trust anybody born with vampiric taint completely.

2) in short, being a revenant/zadruga/etc means that you convert/"produce" units of vampiric vitae (basically magical blood that holds quintessence if you know what that is in the lore, if not don't worry about it, it's a mage thing).

Ghouls are mortals that have drunk vampiric blood. They can stop at anytime and become mortals once again (it's not easy arguably, like a difficult addiction, but it's possible).

Revenants were conceived by at least one ghoul. They inherited some of the vampiric blood of their parent and their body mutated to adapt to that. They produce their own, weak, vampiric blood to survive. Like if the child of an addict produced their own drug to survive.

Gantebein are ghouls hunters. They hate vampires and anything related to it, they use their own vampiric blood to fight evil with evil in a way (like Blade if you will, become what you hate to fight it). If they somehow managed to kill all vampires, they would simply stop taking vampiric blood and become mortals once again.
That's not something a revenant can do, they are tainted forever.