Hi
The grid graph erodes the graph along the 4 axis aligned connections. If using the Collision testing -> diameter setting does not work for you, I suppose you could change the grid graph to erode along all 8 connections.
In the GridGraph.ErosionAnyFalseConnections method, change this line
for (int i = 0; i < 4; i++) {
to
for (int i = 0; i < 8; i++) {
You could alternatively subclass the grid graph and override the method there, in order to make it easier to upgrade the package in the future. See this post for how to do that: Tag Grid Nodes by altitude