Your employer will not be able to obtain any medical information about your condition from your GP unless you have signed a "Consent to Release Information" document. You can also ask your GP to give you a copy of any correspondence they have with your employer if you have consented to information being released.
If your stress has been caused by work-related issues, it is in your interest to cooperate with your employer so that they understand what has caused the problems and can try to put the situation right. If your stress is caused by domestic issues, only you can decide whether you want to share this information with your employer. If you previously have had a good attendance record, and they are a good employer, it may help to share the reasons with them. A good employer will also try to support you although of course, there is a limit to what they can do if it's a small company and a prolonged sickness absence causes them operational problems.