I think there is nothing wrong with a gap year. You can improve your health, do something you won't ever be able to do again or just relax. Some people will take up work at a school, some will travel, some will work and save for the next year. Whatever you do, it will not influence your chances to get onto a PGCE.
I am sorry to hear you have been struggeling with depression and it has effected your grades. I am not entirely sure if you wanna take a gap year between year 2 and 3 at uni or afterwards. I think a gap year during study, an intermission, will be accepted if you are still not well. However, you will lose touch with the academic world. I would rather try and make up the grades in year 3 and then do a gap year and get lots of valuable experience. If you work at a school then, you may even have the chance to complete a school-based programme, such as the Graduate Teaching Programme or similar.