I paid for and downloaded Sim City 3000 Unlimited from GOG. It runs fine.

But just now, like a few days after the download, my AVG anti-virus software just flagged it and offered to quarantine.

Is this normal? Does anybody know what is going on?

seems strange that AVG did that after a few days installed. Was it during a scheluded scan? Can you backup your savegame, reinstall the game and, if avg drop the file on quarantine again, take a screenshot? to see what kind of "maleware" avg has reported

Check the infected file here:

https://virusscan.jotti.org/

And tell what you get. Maybe it's a false positive.

Congratulations, it's a false positive.

Which considering how old SC3KU, (Even the GOG release) shows just how terrible AVG is.

AVG comes up surprisingly often with this sort of thing.

Very probably a false positive.

Although there is a small chance that something else infected the already installed exe - viruses like that exist, even if they are rather rare nowadays. I would run a complete system scan, preferably with several AV apps (like AVG and Malwarebytes) just to be safe.

Tip, stop using AVG and learn about false positives. They're quite common with crappy software like this.