according to wikipedia, their 'free' compiler is GPL without a linking exception. the comparison chart on their page also says it is 'for open source GPL software'.
GPL for an OS is I've thing. GPL for a compiler and runtime is entirely another, since only the latter forces any project you create with it too carry the GPL license.
It is the only Ada compiler available for free, fully updated to Ada 2012, with all the remaining ones are still on their 90's style prices.
https://www.ptc.com/en/products/developer-tools/objectada
https://www.ghs.com/products/ada_optimizing_compilers.html
https://www.ddci.com/products_score/ (frozen in Ada 95)
Thanks to them Ada has become a regular presence at FOSDEM and is being teached at quite a few European university.