Did GPs have to have much training? I was under the impression that in the 19th century it wasn't very difficult to become a GP. asks Dub-Dub.

Well, it all depends which end of the C19 you are talking about, really. It was a century of great change in more or less all areas of society, including both medical science, and the art of doctoring. C18 doctors were looked upon pretty much as craftsmen, not as professionals. (You will be able, of course, to find specific examples to "disprove this statement - however, as a general rule it is true enough.)
The doctors spent a lot of time and effort in the last years of the C17 and first half of the C19 in becoming accepted as professionals. Various professional bodies were formed, The Lancet was published, and medical schools were founded in England, so that one no longer had to go to Germany France, or (God Forbid!!) Edinburgh for training. Examinations were instituted, and eventually, legislation was passed requiring all practitioners to be qualified. Barbers were no longer allowed to practise simple surgery; pharmacists had to have passed exams, as had physicians and surgeons.
So, by about 1860/70-ish, new doctors arriving on the scene had passed exams whereas those who had started practice in the 1820s had not necessarily done so.

The Afghan Wars were in 1838 & 1840, and one might assume with some confidence that Watson was a young man when he went. As he had been to medical school (quite probably Edinburgh, which was (and is) a centre of medical excellence) it is likely that he had taken formal examinations, but is highly unlikely that he had any particular speciality, I think - except perhaps the repair of torn tissue and the relief of fever. A few soliders suffered from the former, nearly all of them suffered from the latter, which killed many more troops than did the enemies bullets.