You mention "responsive" & "stretch" in the same sentence; but in your demo project there is not another layout which would be required for a responsive set-up... so I am assuming you are referring to just using a flexible layout - not responsive. I would think that if we are talking flexible You would want to keep the distance proportionate - i.e. as the overall page size shrunk or expanded the actual pixel distance between images would change but still be the same percentage distance and so maintain the same overall look of the layout.
If You do wish the pixel distance to be maintained at at (roughly 30 pixels) You will need to consider adding responsive layouts.
Anyway, the approach I've taken for this demo is flexible only, following along with (most of) your stated parameters above and your demo project set-up.
Hype Project: projects_responsive_JHSv1.hype.zip (446.6 KB) - Demo here.