I would highly recommend to use the plugin of @bubbleis, because its not natively possible to have an event when RG is rendered, only when RG is loading. (and thats not the same to when its rendered, visible on the page)
This will trigger an event when RG is RENDERED. This is the value which is important for showing and hiding content preloaders oder loading animations.
The only limitation is that it unfortunately only fires once. Only the first time the RG is rendered, not when data changes.
I already submit an idea to implement something like https://blurha.sh/ , but i don’t think that there is any focus on that unfortunately.
Maybe someone can create a plugin for blurhash 
Would save a lot of a time and even if loading times are not perfect it would create a nice user experience.