Is it legal to create music in GarageBand and use it in a podcast

Apple says so ninths document: Using royalty-free loops in GarageBand with commercial work - Apple Support

But even with Apple grading you the rights to use the loops and sounds provided in GarageBand for commercial projects, you may encounter problems, when you try to publish the songs on social platforms. Someone else may have used the same loops before you and claim the copyright.

Yes. While you cannot copy and give away the loops as loops, you are perfectly free to do whatever you want with songs create with those loops.