I would very much doubt the competance of a GP to diagnose dementia properly.
My mother went to her GP and came out with a diagnosis of dementia. Apparently that was based on her walk. What the doctor hadn't taken into account was an accident she had had 12 months prior when she broke her pelvic bone in a fall.
On another occassion, my mother was displaying signs of dementia, which her GP didn't pick up on. That turned out to be due to medication she was given. She had gone into hospital and given digitalis. That should have been a temporary thing but a year later she was still being perscribed it by her GP. When I realised she was still taking it, and that it was that which was causing her dementia symptoms, i told her to tell her GP. The GP was reluctant to take her off the Digitalis, because a consultant at the hospital had first perscribed it. I had to go with my mother to see her GP and I had to tell the GP she would not be taking it any longer, and that the GP must give her some alternative medication. When she stopped taking Digitalis, her mental health improved dramatically.
So no, I would not trust a GP to accurately diagnose correctly, based on what I have seen.