They aren’t an expert at all. A proper evaluation involves several questionnaires and interviews that complement/supplement each other to assess the consistency and veracity of your responses, a consideration of other conditions (since ADHD is very commonly paired with other conditions such as depression or anxiety), and may also involve parents or someone from earlier in your life to check for childhood presentations, if possible.

By your GP’s reckoning I’d be fine because I have a good job and several university degrees, but that’s because I’m smart and could compensate. In reality I was drowning at work and home, and unconsciously self-medicating with copious amounts of coffee, all the while unknowingly masking a fairly significant impairment that I wasn’t even aware was there until I entered my 40s.

It might cost a bit of money but get a proper evaluation done with a psychologist. If the GP won’t help you, get a new one or find a nurse practitioner who has some familiarity with ADHD treatment. Medication is not the only fix, there’s lots you can do without it, and lifestyle changes will help you a lot, BUT it works brilliantly for something like 70-80% of cases so it would be folly not to pursue it. There are also non-stimulant treatments that don’t carry the overblown “drug seeking” fears and that may work for those who cannot tolerate stimulants. Based on the evidence provided, your care provider is uninformed or is not acting with your best interests in mind.