Are wheels really rubber-coated? At the store, they seemed to me pure plastic.
Can be used on gravel?
There are differences between the Best Friend and the higher Best Friend Pro models. The basic Best Friend only has "plastic" - that is, EVA foam coated wheels. Also, different markets can offer different variations of the product itself (there are, for example, different models in the US and in Europe).
For the gravel suitability, I answered your question here. Do read our guides on wheels to understand the topic more.