This little glitch has been a thorn in my side, since day 1 of mapping. Most of my maps, see alot of tau action, and these reflections can become very frustrating. Here's my theory on why it happens.
The Gauss shoots through walls with varying degrees of penetration; therefore, it must calculate the thickness of a wall, and the amount of penetration each shot makes. Reflection occurs when the Gauss hits a brush that cannot be penetrated. It seems the energy is not absorbed by the brush, but rather spit directly back at the player. A simple test map in Worldcraft: make about 5 walls of varying thickness ( the extreme being 200+units), and see at what thickness the gauss begins to reflect.
Reflection can also occur with brushes that arent very thick, if the Gauss shot is a very weak one. Basically, the shot was too weak to penetrate the wall, resulting in reflection.
There is one condition where the wall thickness doesn't matter: If the opposite side of the brush faces the void. For example, brushes that compose the exterior wall of your map. A brush that ends at the void, wont be calculated the same, as the outer faces aren't rendered or drawn.
Therefore, avoid tau reflection, by keeping your brushes fairly thin (128 or under should be ok), or making sure that the outer face of the brush ends at the void.
*even with these principles in practice, some reflection may still occurr. Remember, it is an 'unstable experimental weapon'.