The GNAT Programming Studio is AdaCore's IDE for Ada, C, and C++. It's the most recommended IDE for Ada as others are plugins for editors or other IDEs, such as an Eclipse plugin.
It is available under GPL as specified on their website. The actual source code is available on their Github: https://github.com/AdaCore/gps
Latest stable release is version 17: https://github.com/AdaCore/gps/archive/gps-17.0.tar.gz