Map app for avoiding gravel roads
There is no tool that I know of that solves your problem for you. 

The way I solve the problem of finding out what a road is like, is to 


use Google maps to find a route. 
then use Street View to check the roads at important points. Of course, if the road is gravel, Street View is probably not available. 
if the route is not what I want, drag the route line to another road, and check that.


In addition (I didn't know about these and haven't tried them) there's Gravel map and similar sites that perhaps could be used to see where the gravel is, so you can avoid it.