Why does the Red Skull say Gamora's wrong father?

Thanos isn’t Gamora’s “wrong” father, he’s her adoptive father, albeit forcibly adopted. Adoptive families are still families and she’s been in the adoptive one for longer than her real one. She is a Daughter of Thanos, and was at least for a time a part of the Black Order and one of the Children of Thanos.

Thor: You seem to know a great deal about Thanos.

Drax: Gamora... is the daughter of Thanos.

Thor: Your father killed my brother.

Peter Quill: Oh, boy. Stepfather. Technically, she hates him as much as you do.

_{Avengers: Infinity War}

Using either the adoptive father or the real father’s name would have been fine (though I don’t think we know her real father’s name in the MCU) but it also serves as a set up for the sacrifice. They both love each other, albeit not quite in the traditional sense. By using Thanos in this scene it sets the precedent than Thanos does truly love her for the sacrifice.

Red Skull: In order to take the stone you must lose that which you love. A soul... for a soul.

Gamora: All my life, I dreamed of a day... a moment when you would get what you deserved. And I was always so disappointed. But now... you kill and torture... and you call it mercy. The universe has judged you. You asked it for a prize, and it told you no. You failed. And do you wanna know why? Because you love nothing. No one.
Really? Tears?

Red Skull: They're not for him.

Gamora:!No. This isn't love.

Thanos: I ignored my destiny once. I cannot do that again. Even for you. I'm sorry, little one.

_{Avengers: Infinity War}

Probably because despite everything they think of each other as father and daughter. Atleast Thanos did. And as evidenced by her tears when it seemed to Gamora that she had killed Thanos on Knowhere, she might've cared for him as well.