Equivalent of ImageMagick's -fx in GrahicsMagick?

Hello,

I'm wondering what the nearest equivalent of -fx is in GraphicsMagick is?

I am trying to perform a color to black and white conversion without
using the default formula when changing colorspaces.

In im, I run:

convert -colorspace Gray -fx '0.4*r+0.1*g+0.4*b' in.jpg out.jpg

But I can find no option like -fx

The closest I have found is -recolor, where I specify a full matrix to
perform color translation in the same colorspace. But in the future, I
might want to use something more general like -fx.

Any pointers?

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