Filters/lights and shadow/lighting effect

[denzjos]

I think you mean something like this. A quick example : make a path on a layer, press 'enter' (a selection from the path is made) and fill the path wit the color you like (in this example white). Then 'Select / None' and blur the result (Filters / Blur / Gaussian Blur). Draw an ellips on the bottom of the beam, fill it with the same color you used for the beam and blur the result. I used the 'gradient tool' on a separate layer and set the layer mode on 'colour erase' to blend the light beam a little.