I really don't think (and I am developer, I will need to comply) that anything in GDPR is hard to understand. Treat data from others in same way as you would treat (and you are treating) yours. You are not selling your personal details to 3rd parties, you are not keeping painfull pictures of yourself climbing to garbage bin and doing diving completely drunk, you are not storing them into pastebin or unsecured databas…
I've been looking, and I have found a bunch of contradictory explanations. My best guess is that if you have a disclaimer that says you log IP addresses for security purposes, you can keep your access logs indefinitely. (see: https://community.spiceworks.com/topic/2041760-access-logs-i...)
This seems like the sort of concern that should be clearly addressed with an official answer before the regulation takes effect.