I say she's gone for good. A "Dracula" needs some great tragedy that causes him to turn their back on humanity. With Gabriel, you needed to take away Marie so that he would have nothing to live for anymore so that he could eventually lose his humanity and become the "Prince of Darkness".
Though it makes you wonder what would happen if she did come back right after Gabriel slayed TFO. It could lead to an Alucard like character and who knows what else.