Here's your answer, at least in terms of what I'll call trendiness (search volume). I would say that Gentoo probably lost a ton of users to Ubuntu from the timing (peaked in 2004). But if we look at something like website stats, the data suggest 15,00020,000 unique visitors per day (with an unusual drop in the past year). Since most users don't visit the website every day, I would expect the userbase to be much higher than that.
From the developer perspective, Gentoo's got around 175 "core" contributors (committers every month) while Arch only has about 30 (according to that article in The H).
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