f=2-(x^2+2 y^2)
xmin=-5 ;xmax=5 ;
ymin=-5 ;ymax=5;
Gf=Grad[f,{x,y}] (* gradient of f *)
Plot3D[f,{x,xmin,xmax},{y,ymin,ymax}]
cp=ContourPlot[f,{x,xmin,xmax},{y,ymin,ymax},ContourShading->None];
vp=VectorPlot[Gf,{x,xmin,xmax},{y,ymin,ymax}];
Show[{cp,vp}]