This is what I'm talking about. The idea that "going for a walk and getting sunlight is good for depression" is so universally unchallengable that I must live somewhere terrible if it doesn't work for me.
Book a two week long float down the middle fork of the Salmon river and let me know how you feel after.
Not that person, but I feel the same way they do.
You're still doing it. Depression has a cause, and effective treatment depends on what that cause is. If it is not one of the things that is made better by going outside, this will not be helpful, and the answer is not "well then go more outside".
I don't have depression but I still feel like that'd make you depressed. Just because something is exciting to you does not mean it's meaningful to everyone
There can be underlying anxiety issues that make going out like that quite a stressful event, especially at first, and even traumatic if something bad does end up happening. Or sensory processing issues associated with autism spectrum disorders that make bright sunlight feel unpleasant. Not to downplay the general idea: certainly for most people who feel a little blue, or even crappy enough for long enough to qualify for a diagnosis of major depression, getting out more is likely to be helpful on its own (and that's why therapists generally recommend it). But to the GP's point, the lack of distinction between people with a more ordinary (but quite painful!) set of problems and people with a bunch of comorbidities that they're likely not aware of is really frustrating, especially if they're using the internet to look for ideas and find a bunch of people generalizing the easy fix that cured their depression to the entire population.