As a GP, I can tell you that the dress standards vary dramatically. You will make a better impression if you go smart (even a suit would be fine) with the option to dress it down a bit if everyone else is in jeans and a polo. You will not lose any points for overdressing, but won't make the best impression if you are underdressed.
They are likely to have you sitting quietly in the room with the Dr, but depending on what is happening/size of practice, you may get to see some minor surgery, duty doctor work, home visits, etc. You should also take the opportunity to sit in with the Practice Nurses (looks impressive at interview if you can explain their role and how important they are to General Practice today), the Receptionists and Admin (lets you talk about how important teamwork is), and the Practice Manager (lets you see the part of GP that is running a business-again useful interview fodder). Smile all the time, be polite and respectful, ask if you can make anyone and everyone a cup of tea or coffee and cakes and/or biscuits will probably go down well later you have seen the lay of the land. Usually you will not get the chance to share much more than polite chit chat with the odd patient (there is only 10mins per appointment), but again, be polite, act interested and remember how privileged you are that they have let you sit in on their consultation (they will be offered the opportunity to say no before entering the room). Don't over-question the GP, but do ask any questions you've got. We like enthusiasm, but not being pestered during a busy day. Offer to stay to the end (most of us work 8am to about 7pm as a minimum), though you will probably be allowed to go after patient contact ends, BUT, insisting you need to leave at 5 due to prior commitments will not make a good impression. Also, seeing the admin/paperwork we have to do will give you a fuller view of the job-and again, good interview material. Hope that helps-any questions, just ask.