Alpha channel in c4d rendering?

Alph channels are used to tell the app that reads them whether something is there or not. It determines 'transparency' not depth.

The gradient is missing because it has nothing to do with whether there is something there or not. In you scene you have an object (acting as a fllor that extends two thirds of the way up the frame. So does your aplha channel image. The alpha ends where there is no more object. It show only the lack of a backgraound.

Are you sure you're not trying to render a greyscale depth map? That's an entirely different animal than an alpha channel. Alpha channels contain inofrmation about transparency. Depth maps contain inofrmation about distance from the camera.

Mark Simpson