first you have to change the plot from PLOT_OCEAN to PLOT_LAND (assuming you want flat land), then change the terrain type to grassland:
gc.getMap().plot(88, 47).setPlotType(PlotTypes.PLOT_LAND, True, True)
gc.getMap().plot(88, 47).setTerrainType(con.iGrass, True, True)
the true/false entries...