We report on preparing assemblies of gold nanoparticles with continuous gradients in number density on flat silica-covered substrates. The methodology consists of (i) first forming a one-dimensional molecular gradient of amino groups (-NH2) on the substrate by vapor deposition of amine-terminated silane molecules, followed by (ii) attachment of gold nanoparticles to -NH2 functional groups by immersing the substrate in a colloidal gold solution. Experiments using atomic force microscopy reveal that the number density of nanoparticles on the substrate varies continuously as a function of the position on the substrate. Near-edge X-ray absorption fine structure studies confirm that the nanoparticle number density gradient is closely correlated with the concentration gradient of -NH2 groups anchored to the substrate. We demonstrate that the number density of nanoparticles within the gradient and the length of the gradient can be tuned by controlling the vapor diffusion of silane molecules. In addition we show that this simple methodology can be further extended to create double gradients, thus producing "a valley in nanoparticle concentration".