There is a base Gecko Media Plugin API that allows more specific APIs to be requested. The API is described in code by the text file located at:
<code>contentdom/media/gmp/gmp-api</code>
in any copy of the Gecko source code that includes GMP support. <code>GMPGetAPI</code> returns a void pointer which can be cast to the structure defined for any specific API, such as video encoding or decoding.