Marbleland lets you filter by which version of the game the levels are compatible with when you're searching for levels. If you search for something, you can click the little filter icon on the right of the search bar and choose "Compatible with > Gold," like so.
This will filter for only levels compatible with MBG. Then when you click a level (or download a bunch at once), you can download "assuming you have MBG," which lets you easily put the levels into Marble Blast Gold's package contents by merging with the contents of marble/data.
The three folders inside the folder you download can be copied and then pasted into marble/data in the MBG game files, and then you just merge when prompted.
(Although now that I'm actually doing this, it downloaded a folder called "interiors_mbg", but shouldn't that just be "interiors" since MBG only has one interiors folder?)
You may think this is complicated compared to playing Marbleland levels in PQ, and it is, but in the old days (pre-2021) this was how you had to install and play any custom level you wanted to play, and it was even more complicated because you had to know exactly where to put everything. At least with this method it's a simple copy and merge. Marbleland is great. I hope this helps!