I think I can offer some pertinant experience here.
I projected on to various materials before my final screen was put up. This included the Da Lite High Power material, Carada BW material, dry-wall with primer, and finally the Stewart Studiotek ST-130 material in a fixed screen.
To cut to the chase: The primer ended up being almost exactly the same gain as the ST-130 material, which is 1.3 gain, so above neutral white. (BTW, sometimes two samples of screen material that are slightly different gain can look almost identical if you are projecting images on them. It's hard to get exactly the same
image on both if you are using movie images. A sure-fire way to see the difference is to put up a solid color screen, for instance a full blue screen - some projectors default to this color, like my JVC, when it is projecting no image. If one piece of material has even slightly more gain you'll likely see it pretty easily on a full blue screen).
Once I was projecting on the primer the image was so bright and punchy I was starting to think "who needs a screen?"
Once I put up the ST-130 the differences were: The image was equally bright and punchy, but everything seemed more refined - a perfectly smooth surface seemed to render detail better, contrast, especially intra-scene contrast seemed to improve (probably because the ST-130's gain is directional which would slightly help reducing side-wall light reflections), and there even seemed to be more subtle color detail. Basically, it just seemed a step up image quality wise in every way. I was sort of surprised by the difference, frankly, but was certainly glad in the end I bought a professional quality commercial screen.
I now have my JVC RS20 projector (calibrated by umr) projecting on to the ST-130 screen and it's mind-blowing, perhaps the best projected image I've seen.