AdaCore maintains the GNU compiler as well. As a result, the GNU and AdaCore compiler (Community and Commercial) are essentially the same. The Ada community in fact just calls them all GNAT and then specifies further if it's relevant. The main difference is that GNU can lag behind a bit historically, but recently there has been some initiative from inside AdaCore to tackle that issue and they've done a great job.

Grab whichever is easiest for you to get started with. It's a weird and ultimately meaningless situation license-wise. If and when licenses matter, grab the GNU version. Unless you're doing something that requires a support contract anyway, then you'll end up with Pro.

I really wish AdaCore would do something about that Community edition. I have no idea what purpose it is supposed to have. In my opinion, right now it's mostly a thorn in the communities side. All it does is create a bunch of hesitation and confusion about the licenses and compiler capabilities for newcomers. As a result, before people even start with Ada they're getting deep into license discussions which is just a wonderful first impression. And worst of all, it's all basically for nothing because they're at most slightly different versions of the same software with a different license.