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There might be a better fix involving a clever use of BlendModes... I think dstOut is what you want here, with an alpha transparency mask (using alpha instead of the background color).
https://master-docs-flutter-io.firebaseapp.com/flutter/dart-ui/BlendMode-class.html
Then you would be able to handle a background image.

Do we want to go with the blendmode approach, or do we prefer the approach here of drawing a coloured gradient over the top?

I don't know... I've been really busy lately and couldn't touch the code again.
What do you guys think?

To me, the blend mode approach seems like the right (more general) one.

I tried going with the blend approach for a bit (left #16831 for tracking) though I'm blocked by #14224 in the process. Nevertheless, I think it's the right approach too.

It sounds like we're going to go with the blend approach so I'm going to close this PR for now. Thanks again for your contribution. If you want to try doing the blend approach and have time to do so, don't hesitate to submit a PR for that. Otherwise we'll get to it eventually, see #16831 to track the progress on that. Thanks!

@Hixie Do we still want to work on this or are we keeping the linear, white based gradient?

@jslavitz we can use your help with this one. Though with a slightly different approach using different blend modes instead of drawing a white or colored screen.

#16831. I suspect the upstream blocker on that one is solved but would double check.

@xster Looks like that upstream blocker has indeed been resolved. As far as fixing this issue goes then, do you have a specific idea of what the blend modes should look like? Do you mean something like additive/multiply or something different? I think it could make sense to let the user optionally choose a blend color that is set to white by default.

Since it's still an iOS picker, the gradient color is never different from the background color so we shouldn't let the user pick 2 colors.

For the blend mode, see Ian's comment at #16331 (comment).

Either dstIn or dstOut should work. It could just be a matter of specifying the backgroundBlendMode on BoxDecoration above the LinearGradient.

@xster Do you know of any examples using the dstOut blend mode with the boxDecoration? Things aren't working as expected when I try to use it. The tests for boxDecoration are also pretty sparse.