This message is generated when the value of the GridLines option does not match an expected form.
The value of the GridLines option is expected to be None, Automatic, or a list {xgrid,ygrid} specifying the grid in each direction.
Each grid line is described either by a number giving the position of the line or by using a list {pos,style} indicating the position and style of the line.
The value of the GridLines option can also be a function that returns a grid specification when applied to the minimum and maximum values of the plot range.
Off[message] switches off the message; On[message] switches it on. For example: Off[GridLines::grid].