After looking at a few videos of Game Builder Garage, I'd say it's infinitely more complex than Mario Maker 2, but I'm guessing nowhere near things like Unity, which I never used.
Confirmed that you cannot browse other people's creations; but maybe that's for the best, considering that you can add custom text and then they can ignore inappropriate content and say only those looking for it will find it. Also, it looks like you can add custom textures and one game even had a custom loop of music!
You can skim through this 20 minute video for some examples:
https://youtu.be/i7ocaD8vXFg Ryukahr has been treating this as if it were an extension of Mario Maker from the very first video, as did a couple of other Mario Maker streamers.
Considering that the game has been out for like two days, I'd say these examples show a lot of promise, especially taking into account how garbage early Mario Maker 1 levels were and where they got.
As with other things Nintendo neglects, this game would probably benefit from ongoing DLC or updates to keep it fresh and add options where they are found lacking, and it could become a big thing.
I'll keep an eye on this game.