Certification process
It depends on the version you hold. An original with a wet-ink signature or an embossed seal can go straight to the FCDO; a photocopy or print-out must first be certified by a UK solicitor or notary public.
Document requirements
Wet-ink GMC-registered practitioner signature → direct; copies → Tier 2.
Digital e-Apostille
Possibly — an e-Apostille needs a PDF of your doctor's letter / medical report that has been electronically signed by a UK solicitor or notary public. If you only hold the paper version, you'll need the paper apostille.
The Hague Convention
An apostille authenticates a UK public document for use in countries that are members of the Hague Apostille Convention. Destinations outside the convention may need further embassy legalisation instead.