i took two "gap" semesters, but instead of focusing on my mental health i went ahead and worked part time and took a few classes at a community college...unsurprisingly i still struggled when i went back to school
i would say if you do take a gap semester for your mental health, ONLY focus on improving that. go to a therapist, rest, do light exercise, and develop healthy coping mechanisms for whatever things you were struggling with doing school full time.
i so wish i prioritized that, because i feel i would've suffered a lot less during my undergrad.
cons would be: interrupting your studying momentum would be the only thing i can think of? there is a stereotype of students taking gap semesters and just not returning but i know of more who did return than didn't
because i took two semesters off i was a year behind my friends and that kinda sucked but more because i was sad about how things played out for me
good luck! you deserve rest and good mental health! you're still very young so take the time you need!