Xcode 12.0 GM (12A7209) - how to stop app store version updating it?

Outside of disabling updates globally, there is no way to avoid the MAS sniffing your applications and blacklisting individual apps.

I think it's not uncommon for the store to ignore non-MAS installs, even when the build number is the same.

Pick one process and stick with it.

I assume KMT's answer is satire. Lol. Of course Apple specifically URGED developers to download the GM version so that we could submit our app updates in time for iOS14 public release.

Sorry, is that a new question, or?

"Pick one process and stick with it." - I wish it was that simple.

Apple's developer website simply points to MAS for non-beta releases. But of course betas are not available from MAS, so had to download and update these from the developer site.

So, KMT, unfortunately your answer is just not realistic, and in any case doesn't address the specific situation I (and I am sure many others) are currently in.

Further information:
1) The Apple developer site now points downloads of current Xcode version (12A7209) to MAS - so it is indeed the same build as Apple provided as GM, and that I had already installed.
2) When I let the MAS proceed with its update of Xcode, it downloads the entire update but then immediately fails with a message "Xcode could not be installed. Please try again later.".

So my question still stands. How to rectify this situation?

I can’t help you with the App Store side of this (sorry!) but I can you with the following:

Apple's developer website simply points to MAS for non-beta releases.

That’s true, but you can get to a downloadable Xcode 12 by clicking on the More button. That takes you to this page, where you can download Xcode 12 GM seed. I did this today and it yields the GM build (12A7209).

Personally, I always download from the developer web site because it avoids this problem and also lets me archive away the release for fast access in the future.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@apple.com"

Based on answers given here, this is how I resolved this issue:

Note - I had previous renamed the Xcode.app to Xcode11.app before I downloaded Xcode 12 GM from Apple developer site, knowing that once unxipped it would want to name itself Xcode.app. After doing this, apparently Mac App Store was still pointing to the Xcode11.app file as the one it "owns".

1. Made a copy of the old Xcode11.app and named the copy Xcode11p7.app (in case I need to use the old version again sometime)

2. Deleted the Xcode11.app file - this immediately caused Mac App Store to remove its update prompt for Xcode

In future, I will only download Xcode versions and updates from Apple Developer site.