Converting grey layer to transparent

[locetina]

I have a light grey layer containing shadow (Question-Start.png) exported from a 3D application. I can use it over a background layer using a Multiply blending mode. But I would like to convert it to a transparent layer without losing any shadow information, similar to the example in the attachment (Question-wanted_result.png). Any ideas on how to do that?

[Old Bruce]

I would look into getting the 3D application to export the shadow as a PNG with transparency.

Failing that I would use one of the PNG's RGB channels to make a spare channel and then invert that. Finally use the inverted spare channel as the PNG's alpha channel. 

 

[Lisbon]

Blend options can be an option.