Re: "(or possibly a note from your GP)"
GP = General Practitioner.
It's basically a doctor who works in a standard surgery. These days, they very rarely make house calls (they don't come to your home). If you work for a company which insists on a doctor's certificate when you're off sick, then you have to visit your doctor when you are sick and request a certificate. It will give the details of your illness and how many days your doctor recommends you stay away from work. If your doctor doesn't think you are ill enough to miss work, you won't get a certificate. With many companies, if you don't get that certificate, you won't get paid anything.
In the UK, if you want medical treatment (non-emergency), you register at your local surgery with a specific doctor. That's why the piece says "your GP".
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.