What is the GraphicsMagick equivalent of ImageMagick's +opaque?

Extracting a specific color (red) from an indexed BMP in ImageMagick goes like this:

$ convert input.bmp +opaque "#FF0000" red.png

Trying it with GraphicsMagick fails as it appears to not support +opaque, only -opaque.

How does one do the equivalent with GraphicsMagick? Perhaps there is a cheatsheet for migrating from ImageMagick to GraphicsMagick?

Many thanks.