Synthclipse is licensed under very permissive Eclipse Public License v 1.0. You can freely use, modify, copy and distribute the application or modified versions of it.
Some assets (textures, music) and shaders included in Synthclipse are licensed under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. They are explicitly tagged as such, either in comments at the beginning (shaders), or in README/LICENSE files contained in their parent directories (assets).