It is open sourced under Microsoft Public License:
https://github.com/BabylonJS/Babylon.js
According to the Free Software Foundation, it is a free software license but not compatible with the GNU GPL.
According to the Free Software Foundation, it is a free software license but not compatible with the GNU GPL.
That doesn't mean much, the GPL (v3) is incompatible with itself (v2). Not a GPL hater, just pointing out that it's not a big deal for licenses to be incompatible with the GPL.