Merlin uses Asus GPL (public codebase) and makes his improvents (security and bug fixes, driver updates and many features including the possibility to run your own scripts) based on it. But only for Asus RT routers.
GNUton uses Merlins GPL and adapts it for DSL-AC68U which is very similar to RT-AC68U, most hardware is the same beside DSL-part.
@UK Sentinel: Would be really interesting for me to have his knowledge 😉
I can recommend to use their enhanced and updated firmwares, they are public and everybody can use and compile it on his own as it is or add your special changes before.