Is the version of Spore offered by GOG actually playable?
I know the original required access to no longer functioning servers....

Pretty much all games on GOG are DRM free, so yes. It's playable.
Of course, if you want to access online features, check this out: https://support.gog.com/hc/en-us/articles/115002333209-Spore-How-to-get-access-to-online-features?_ga=2.21898750.41440512.1590889764-1689885120.1573235895&product=galaxy

I have purchased spore here and I am able to download and connect to the servers as of just a few days ago. I think it helps that I had purchased Spore years ago and had an account already set up. I just used the same user information and I was off and running.

Are you thinking of Darkspore? That was a Diablo-esque spinoff that was rendered completely unplayable when the servers went down. The original Spore's servers are still up, as far as I'm aware, and the game should play without access to them (you just won't see other people's creations pop up in your game).