Don't listen to Gong all that often anymore but they're still a favorite (as are the rest of the major Canterbury groups) and I actually pulled out "Magick Brother" (my favorite) just the other day. Those Daevid-era albums are all pretty great (less of a fan of "You" than the others though). Not familiar with the stuff under the Gong name after he left but if you like those main ones, also check out "Live Etc" (killer live comp with versions that beat the studio versions), "Pre-Modernist Wireless: The Peel Sessions" (recorded 1971-1974) as well as Daevid Allen's "Bananamoon" and Steve Hillage's "Fish Rising" (both of which feature Gong members and seem like "lost" Gong albums).
Those first couple of Group 1850 albums are cool (I thought they were the only ones but guess they did a 3rd) and fairly unique. Got them both maybe a couple years ago but only listen occasionally so I still feel like I need to absorb them. Prefer the first one- more creative and unpredictable- but the 2nd has some killer acid guitar.
That Älgarnas Trädgård is great. Just listened the other night (great smoking music) Was a bit underwhelmed when I first heard it (because of its reputation as an obscure masterpiece ) but it quickly grew on me. Never thought of the post-rock connection (basically given up on post-rock myself) but you're right- it's there. Still a uniquely timeless album though. The 2nd album "Delayed" is less unique and more in the space rock vein but almost as good in its own way.
I need to spend more time with those Ultimate Spinach albums myself. Good shit, especially the first one. One of those bands that sounds like a combination of like 5 other bands (in a good way).