x3 - 2xy + y3>0
The way to do this is to define a function that has a constant value, if the inequality holds true, and has an indefinite value, if this is not the case. The value of 1/0 is quietly ignored in gnuplot, thus we can define our function as follows
f(x,y)=(x*x*x - 2*x*y + y*y*y>0)?1:1/0
where we used the ternary operator, which always has the form
i.e., returns 'B', if 'A' is true, and 'C' otherwise. (Incidentally, the value 'B' can be a ternary operator in itself, and in this way, multiple conditions can be imposed.) In the function above, f(x,y) = 1, if x3 - 2xy + y3>0, and f(x,y) = 1/0 (undefined), if x3 - 2xy + y3<0. Once we defined our function, we can plot it in the usual way. The complete script and the figure is given below.
reset f(x,y)=(x*x*x - 2*x*y + y*y*y>0)?1:1/0 unset colorbox set isosample 300, 300 set xlabel 'x' set ylabel 'y' set sample 300 set pm3d map splot [-2:2] [-2:2] f(x,y) t 'x^3 - 2xy + y^3>0'