Morgana turned Gwen evil witb her ways...

I think that Morgana has somehow brainwashed Gwen by taking her to the Dark Tower, whereas i think that it was more of a natural occurance with Morgause and Morgana is her sister so they would have a strong bond.

No. Morgana wanted to kill Uther by S02. When Morgause asked Morgana if she actually wanted that to happen, to become an ally, and would do what was needed, Morgana willingly said "yes". So, Morgana willfully agreed to kill the king and undermine the kingdom.

Gwen on the other hand was exposed to the mandrake root which allowed Morgana to gain control of Gwen's mind. After Gwen was brainwashed by Morgana, she exerted her control over Gwen to commit "evil acts." Had Morgana not used the mandrake, it's not very likely that Gwen would willingly try to kill Arthur or undermine Camelot.

I don't think she's done anything 'magical' as such to Gwen. I believe that Morgana over-exaggerated about the bad things that Arthur may have done to her. And because of the horror Morgana put Gwen herself through, she thinks that is how Morgana felt.

I think morgana did brainwash gwen and then forced her to always wear black when she sees her