Rendy is a bit of a rough start for a newbie. It gives you a lot of power, but it will take a lot more work to draw a cube rotating on screen than otherwise. Gfx-hal is a foundation of Rendy, so learning one or the other won't be exactly easier or harder. Vulkano is the api they follow, so understanding that will help you understand rendy and gfx-hal (as the question becomes: how did they name this vulkano functionality).
Still it's hard to understand what is the value on these things without context, which is why some people prefer top-down (start with a more specific problem and then specialize).
Here's a rundown of the layers:
So, if you want to learn bottom-up I'd recommend to go Vulkan->how to do the same in gfx-hal->Reusing the cool functionality of rendy instead of rolling your own
If you'd rather learn top-down (I am not assuming how much high-level graphics experience you have, but it really helps to gain context), start with amethyst -> how amethist-rendy is plugged into amethyst -> how amethyst rendy renders things -> vulkano api + why gfx-hal and rendy work the way they do.