Create heat maps in GPU.

I am struggling with the following.
I need to plot a heat map of size say 1000x800. The values that I need to plot are between 0 (green) to 1 (red). As value at a pixel increases I the point becomes more and more red. if value is 0.5 then the colour of the pixel will be 0.5green + 0.5red.

Currently, I am calculating these values for each pixel in GPU then transferring those values to CPU and writing them in a file. Then using gnuplot to create the image.

I was wondering instead of doing this, can I write each pixel of an image in GPU and then write that image.