That's the old HL1 method, which is completely out dated. Even the volumetric light models look pretty shit compared to other game engines.
So theoretically, as Tapp said, you could create gradient textures and assign them to the faces of func_illusionary brushes... but you're better off either approaching it from a different angle or asking someone to convert what your desire into a model. It's not exactly a difficult request off of a modeller, assuming you can find a half decent one willing to collaborate (which is a totally different story).