Well, I've made many stupid mistakes in my time on BirdForum, but even I am not stupid enough to challenge Lou Salomon on a gull ID. So, congratulations on finding this bird.
I live in Nara, which is a long way from the sea, and so I rarely see gulls, much less spend days looking for rareties as some do.
But a couple of years ago, I bought Olsen's 'Gulls' just for fun because people who are into gulls seem to get really into them, so I thought I'd explore what caught them up. As I said, the illustrations in Olsen (by Hans Larsson) of flying Glaucous-winged which have as much brown on the body as yours' (Gerben's) have completely strong black bills; however the illustrations of standing birds are much more ambiguous, I agree.
Back in 2005 which is the last time I visited Hokkaido at a season when these species would be around, I took the first two attached photos at Lake Furen (a sea lagoon) - actually the birds were not far apart on the shore. At that time I had barely started birding, and didn't know about BirdForum. I ID'd the black-billed one as Glaucous-winged and the one with a pale bill with black tip as Glaucous. Again, this was because I thought that Glaucous-winged younger birds had an all-black bill and Glaucous not. But maybe these IDs were wrong - please let me know if so.
Therefore, I was just
surprised that Gerben's photos were Glaucous-winged when only the bill tip is black, and I'm not sure the photos are clear enough that you can see a pale tip to the bill or not.
Another reason for looking at this thread was that at the end of last December, I spent a few days in eastern Hokkaido, just for a general break as well as birding. We found quite a few Glaucous Gulls (ID on upper wing pattern in almost all cases), but no Glaucous-winged. The only outlier was this (third and fourth photos) young bird (by itself, no adults around), which seems to have a lot of black on the bill. I put it down as Glaucous because all the other birds we'd seen were. But maybe it isn't - or given how poor I am at gulls, maybe it's some other gull completely than GG or GwG?
Anyway, best wishes.
Here in Nara, Japan, where I am, global warming is speeding up: the cherry blossom started 16 days earlier than average, and full blossom was last weekend (twenty years ago, 8th April or so). Last year and 2019 were the equal two hottest ever years. This year, 2021, January was not in the top ten hottest months, but was the sunniest January on record. February was the hottest February ever - at an average temperature (over 24 hours, over all the days of the month) of 7ºC it was 0.7ºC hotter than the previous record, and 1.1ºC hotter than 2020 (6th hottest) and 2019 (7th hottest). Today (with two days left to go), March is 0.5ºC hotter than the previous record (2018), and 0.8ºC hotter than 2020 (third highest) and 1.6ºC higher than 2019 (fifth highest). So, I don't know why they are talking about keeping global warming to under 1.5ºC above 'pre-industrial' levels - it's already more than 1.5ºC hotter than ten years ago where I am. This is surely going to affect birds - my friend claims to have heard Narcissus Flycatcher singing ten days ago, which would be a month early.
