I am replying for the opportunity to learn something and to show my ignorance ...
My understanding of all channels is that they are greyscale images. Red is a grey channel, blue and green also, for each layer. A layer mask is a grey channel attached to that layer only. Alpha channels are a storage device for those greyscale images that are not attached to anything. They can be used to create a selection and then create a mask from the selection, or in Apply Image or Calculations and I am sure in other ways ...
To create a gradiated Alpha mask i guess you would (I havn't done this before) create a layer or a mask, fill the layer or mask with a gradation with the gradation tool, Select the luminosity of that layer or that mask, save your selection as an alpha channel. Or maybe you can just create a new channel in the channels window and paint a gradation on the image???
If you had multiple colors the do different things you would need a channel for each seperateley if you wanted to affect the things they did seperately, or create a selection based on the combined colors and save that selection as an alpha channel.
I am sure I got part of this right, and for any I got wrong we will all learn double