Serin9X: Ok so I've been monitoring it over the last few days and I've come to the following conclusions:
1: I'm not using any plugins that didn't come with the client. I have connections active to GOG, Epic, Origin, PSN, Steam, and Uplay. All these plugins came with the stock client download.
2: The Steam plugin in particular is CPU heavy. This means that any time the Steam plugin is working (not crashed which does happen quite a bit) it's drawing something like 10% CPU all by itself. I narrowed this down by tracking the python processes the Galaxy client is using to operate its integrations. All of them spike at load but most of them calm back down after a minute or two. Steam not so much.
3: I don't know what precisely caused the 30% CPU draws I was seeing originally. Probably a combination of factors, but I wasn't tracking anything in particular at the time so I can't know for certain. I do know that the CPU usage while the client is active is quite high, and much higher when another client is active at the same time, which for a launcher that launches launchers, this is not super unreasonable.
I want to also mention on the subject of the Steam plugin, my Steam library is enormous (almost 800 games). I wouldn't consider it unreasonable for a library of that size to take some time to scrape through. Depending on how often Galaxy is doing that, I could easily see it causing high CPU draws.
Hmm, there seems to be potentially bug about GOG Galaxy keep generating more python.exe. Never got around to test it. But sill, hard to image GOG Galaxy generating much more CPU time than Steam client itself.
How long (if ever) have you been re-initiating your GOG Galaxy? Either by closing its app on your tray icon (id on Windows) or after turning off and on your PC if ever, seems nobody does that anymore, which is okay :) )