The Gnat in RAF service was fitted with a Folland seat not an MB one. It did not eject downwards. I can't think of any ejector seat which did. I can't imagine that the human brain and eyeballs would be able to withstand a peak G force of around -25!
Since that Gnat left RAF service in the 1970s I imagine it might be quite difficult to find qualified tradesmen to keep the Folland ejector seats serviceable with safety but I don't really know.