I use mapmyride there's a dropdown that let's you add height trace. Takes a few goes to get used to it, but I've used it for years now. Gradients are colour coded.
My only criticism would be that as the ride length goes up the gradients get smoothed and you may not see the max instantaneous.
As to Shetland, I would describe it as rolling, and windy, there are very few of what I would call steep gradients, and those are generally short sections
