GNU Radio is described as 'free & open-source software development toolkit that provides signal processing blocks to implement software radios'. There are more than 10 alternatives to GNU Radio for Linux, Mac, Windows, Android and BSD. The best alternative is CubicSDR, which is both free and Open Source. Other great apps like GNU Radio are SDR# (Free), Gqrx (Free, Open Source), Universal Radio Hacker (Free, Open Source) and inspectrum (Free, Open Source).
LuaRadio is a lightweight, embeddable flow graph signal processing framework for software-defined radio. It provides a suite of source, sink, and processing blocks, with a simple API for defining flow graphs, running flow graphs, creating blocks, and creating data types.