I have pretty severe ADHD, and have always held “professional” jobs as a software engineer. Your GP, ironically, gave you unprofessional advice. The fact that he didn’t even see the need for testing was really unprofessional (since when do doctors diagnose by eyeballing?).
There is no stereotypical person with ADHD. We’ve learned over the years that it shows itself in many different ways, sometimes in quite hidden ways.
As another example, one of the reasons it took me 46 years to come to this diagnosis is because I have an impeccable sense of time. I never miss a scheduled event. in fact, I always show up early. I can even wake up at a pre-determined time without an alarm just by declaring it just before bedtime. I’m also a very accomplished amateur drummer. You can set a Swiss watch to me. Part of the reason for this is my upbringing, where honoring time commitments was high up on the expectation scale, and the other is because time management is something I hyper-focus on and always have.