If I said I am going to the building where my GP works, is it doctors or doctor's?

Also some people refer to a doctor/doctor's as a practise - is practise or practice correct?

Many thanks in advance!
I'd say "I'm going to the doctor." This completely avoids the possessive issue, and it is the most common way.

You have only one doctor, so if you want to say you are going to his/her location, you could say..."I'm going to the doctor's (office, building, practice)." If you're going to see a group of physicians, then you would say "I'm going to see the doctors."

I hope that helps.
Thanks very much - very useful :-)