The source code is currently licensed under GPL 2.0. The NDI SDK is under a proprietary license (NewTek's EULA) but is not included in the source code of obs-ndi.

However :

• the code of obs-ndi makes use of the headers provided in the NDI SDK
• the code of obs-ndi is linked at compile-time to the NDI library
• the compiled releases of obs-ndi are distributed with a copy of NewTek's NDI libraries

So in it's current state, the plugin is not really GPL and hence can't legally be used within OBS Studio.