Able to practice medicine?

There's not really enough information provided in your post. Your handwriting is unlikely to be worse than some doctors that are happily scrawling illegibly in patient notes in hospitals around the UK. There might be some consideration about you being immunosuppressed and exposed to patients with unpleasant infections. *I'd be surprised if this is insurmountable, though, and would imagine that you would just have to take additional precautions, e.g. the US advice here.

It will be an occupational health issue more than a GMC one. There are doctors with organ transplants, for sure, and doctors with tremors. Can make some of the fine motor things like taking bloods tricky, but I can't say for sure whether that would turn into a major issue.

There are loads of alternatives to being a surgeon apart from being a physician.

It won't affect your application, although they may request an assessment. They cannot discriminate based on disability, so be open and honest about it and if interviewed, perhaps suggest an alternative.

E.g. what happens if you have a serious tremor while working? Response: I will make my colleagues aware that sometimes I have episodes of tremors, I will be sure to put no patients at risk, I will reassure the patient that its a temporary spasm and that I remain competent.

I guess my only option is to be a physician, instead of a surgeon.

What do you suggest?

*

Anaesthesia, oncology, sexual health, A&E, general practice, the medical specialties (cardiology, dermatology, endocrinology, gastroenterology, care of the elderly, respiratory, neurology, palliative medicine, etc etc), occupational medicine, paediatrics, pathology, psychiatry, public health...

Is your tremor a permanent effect of one of your drugs or something that could subside if the drugs were changed in the future? You are a long way from having to choose a specialty and lots of things could change (including development of immunosuppressants that are less neurotoxic!) by then.*