First of all, make sure at least one node is selected in your diagram. The color gradient will start at this node.
Use "Tools" -> "Colorize Graph", on tab "General" choose "Graph Distance" as "Highlight Feature", on tab "Graph Distance" disable "Rainbow Interpolation", use "#ffc4c4" as "Start Color", "#c4c4ff" as "End Color", and "200" as "Maximal Distance".
Note, this will only result in a top-down color gradient if your graph is arranged using "Layout" -> "Hierarchical" or "Layout" -> "Tree" with "Orientation" set to "Top to Bottom".