Hi, dan
Unfortunately, it is not available on windows officially. But don't be sad I am having a good news for you. Recently I was searching it for the same software and look what I found.
I found a website called https://garagebandforwindows.me. You can find and sort out your all queries there. A nice article has written about you can "How you can download and install Garageband for Windows".
All the best!