I mean, obviously? Gabi attacked Kaya out of bigotry, Kaya attacked Gabi because she knew that Gabi PERSONALLY killed a person she cared about. How is that not completely different lol
They're really quite complicated situations but if you boil them down to those elements of course it's easy to say one is justified while the other is not. I would say you could reduce it further though and saying "attacking someone who is not an active threat to you is unjustified". Beyond knowing they both did something wrong I would look at the underlying reasons for
why they chose that action and the circumstances surrounding it.
To which I would say Gabi is a captured 12 year old prisoner of war, who is in the middle of an escape for what she sees as her own freedom. She isn't killing Eldians as a means of penance against the race in those exact circumstances (even if her intent is to do so through militant means later), her bigotry wasn't defining her actions as much as her fear was. Her volatile and violent nature made her strike out, her bigotry wasn't the force behind it.
Kaya though is not in any danger, nor can she mistake her circumstances for danger. The person she is attacking is confined and trapped by numbers, with no immediate threat to her. She might be "personally" associated with the death of Sasha, but Nicolo, her father and mother have already deescalated the situation and made it clear there's no threat or wish for harm. Kaya, who isn't shown to be volatile or violent, nonetheless makes a dash to kill Gabi not to save Sasha, but satisfy her own desire to have Gabi's life ended for essentially belonging to a military.
So I agree their situations were completely different, but it's a completely different take than yours. In the end they were both saved by outside forces, they each need someone else to be a moral compass directing them to what is right. But they were each just as guilty if not for the help of their friends.