Gabi may not know much about DID and might feel that Abigail made all of that up after she killed Andre as an excuse to pin it on her. I can give her the benefit of the doubt because she has been through a lot, sent to prison for a crime she did not commit, taken away from her child, beaten up and still recovering from her injuries, and she's still in prison while Abby is running around town free. At the very least, Abby should be under Marlena's care or in a psychiatric facility.
Salem residents are just beginning to see Abby's mental illness. However strangely the alters acted, people in Salem thought it was Abby doing all of these things. Until her mental illness is fully understood, there will probably be a lot of misconceptions, but more than anyone else, Gabi has a lot to process for what she has been through.