A quick view of Gelli print backgrounds I used recently in my Fake Journal (days 14 and 15). I'm glad I took a picture of them beforehand. In the case of Day 15, the background almost completely disappears behind layers of gouache.
I found that the newsprint paper worked better than the baking sheet. I should have thought about it really - baking sheets are designed so that cookies don't stick, and the same thing happens with paint. Whatever it is that they use (all good, I'm sure, as it's used for food after all), it's very hard to paint over it - it took a good few layers of gouache to cover it. On the other hand, it's great if you want to lift the paint in order to reveal the original background, which is exactly what I did in a few places.