density = ListDensityPlot[tableMap, Mesh -> False,
    FrameTicks -> 
     {{{0,"-2"},{xGridPoints/4,"-1"},{xGridPoints/2,"0"},
  	{3 xGridPoints/4,"1"},{xGridPoints,"2"}},
  	{{0,"-2"},{yGridPoints/4,"-1"},{yGridPoints/2,"0"},
  	{3 yGridPoints/4,"1"},{yGridPoints,"2"}}},
    ColorFunction -> colorMap];