Create and composite linear gradients over image

[lukestoward]

I have been experimenting with govips and I can't seem to figure out how to create a linear gradient of black fading to transparent that I can composite on top of another image.

The equivalent of running these vips commands:

$ vipsheader nina.png 
nina.jpg: 6048x4032 uchar, 3 bands, srgb, pngload
$ vips grey t1.v 4032 6048 --uchar
$ vips rot t1.v t2.v d270
$ vips bandjoin "nina.png t2.v" x.png

It appears this is possible with the php-vips package as John Cupitt recently described how in this thread (near the bottom), however the php API is quite different to the govips API.

I've also seen a similar question here where it's possible in Python.

Is anyone able to suggest a way that I could achieve this? I recently raised the question on StackOverflow, but I've had no help thus far, so I would be super grateful for any assistance.