Is Terraria compatible with MacOS Catalina (10.15)?
With the latest version of macOS, Catalina, games not updated to 64-bit no longer work at all.

The product page says that it "works on Mac OS X 10.9.5+". I assume this means that it has a 64-bit OS, since it wouldn't run on recent macOS versions otherwise. Plus, GOG has removed Mac support from a bunch of (mostly older) games that had only 32-bit versions; the fact that Terraria wasn't one of them is a good sign.
At any rate, you should be safe either way if you buy it -- if the only Mac version is 32-bit for some reason, it would be GOG's fault for not making it clear somehow on the store page, and you'd easily be eligible for a refund. :)

Hi, thanks for your reply.

I looked at another popular site and the site says:
This product is not compatible with macOS 10.15 Catalina.

And now I think that this version also does not work on MacOS Catalina.
A refund is good, but why buy a obviously not working game.

Maybe there is news about updating the game with a 64-bit code for MacOS?

Well, supposedly, the game is still being developed (even though the last update was more than two and a half years ago), so -- assuming the other site you checked is right, and it's currently 32-bit only -- maybe they'll update all the versions to 64-bit builds when (if) they release another update.

Sorry I wasn't able to be more helpful.

1.4 is slated for a January release. I haven't been able to find any dev comment on whether that would include a 64 bit version, but from what I can recall it might not be an easy task. Changing a game developed as a 32bit application to 64bit is usually not just "flipping a switch" once you're several patches past launch.

And, in this case, more than 8 1/2 years past launch, by which time probably the vast majority of their sales are already in the past.
I wasn't saying it was trivial; just pointing out that if a rebuild were to happen, it would probably be released at the same time as a content update.
(On a related note, just earlier I read this Soldak Entertainment blog post that someone linked to from the GOG release thread for their latest title, Din's Legacy. It's kind of interesting to see the hoops that devs could have to jump through to keep supporting Macs going forward. I imagine Terraria sells much better than all the Soldak titles combined, so it might be more financially worthwhile for Re-Logic than for an even tinier studio, like Soldak, to keep their game running on the latest macOS updates -- especially considering they only have the one game -- but they still might well decide that it's not worth the hassle.)

Current version (1.3.5.3) runs fine on Catalina.

I can confirm that 1.4 also works on macOS Catalina.

However, it takes much longer to launch than 1.3.5.3 did, and the Video settings are now much less macOS-friendly.