I think you misunderstand. The integrated Google Analytics feature on Itch analyses visits on your game page on Itch.io. This is before you download the game. This happens as soon as you click on a link to the page on Itch. I don't have any power to ask a person permission before they enter my page.
Anyway, the question was not what I should do to make it legal. The question was whether this is what the GDPR is about or not. It turns out it is not, as what I wanted to do is seen as an exception. (n° 5 mentioned earlier in the topic)
I'm not trying to bypass the law in any way. If it's illegal, I'm not doing it.
