Just curious, are there games whose patches I can only download through GOG Galaxy or are all game patches available through browser downloads?

Well, I don't want to raise the galaxy protectors here, but apparently there is at least one (and it's not something anyone has really checked n depth) - riven which has a slightly better version via galaxy than via classic.
I can't see a way that you would be able to check unless you had galaxy installed and downloaded each bit and compared them exactly, although Magog might be able to give some information. I would though say it's unlikely a patch is available via G rather than other, at least after a small delay as in most cases it would get flagged up, although updates are totally screwed on the site in general.

If they haven't sync'd up between Galaxy and manual download within a few days, contact support to report the problem. It could be an actual glitch they need to fix, or it could just be the normal expected time delay between patches landing on Galaxy (specifically designed to make the patching process faster from developer to installed game), and the traditional 1990'ish good ole standalone installer method that's a FPITA behind the scenes. :)

It depends what you mean by patches.. any proper patches released by the devs should be available through Galaxy or the website (with a small delay sometimes for the offline installers) but Galaxy games do sometimes get Galaxy specific updates that the standalone installers don't (or at least don't get notified about or get new version numbers for), but these are only to fix or add features for Galaxy so if your not using it they should be of little interest.