Mac -- I grow this stuff - it came free as a few stray strands in an order of 'pods from Indo-Pacific Sea Farms. I didn't know what it was by name at that time, but its thin structure and deep red color immediately indicated that it was a deep-water, low light type of macroalgae. (Plants that need a lot of light have broader, brighter, more transparent "leaves" to collect the rays of the sun.)
Even at the bottom of a 75 gallon tank with only 440 watts of mixed Actinic and full-spectrum VHO, the light was too bright for it, and after a week or so it was getting to be orange and stringy -- a sure sign of too much light. So, I secured the remaining strands loosely together with polyester thread, anchored it to a small bit of rock, and tucked it well back UNDER the reef. It now thrives there in indirect, very suffused light, and has actually rooted to the rocks in some of the dimmest recesses. So, while you've tried all sorts of very bright light for growing it without success, you probably just need to try a lot LESS light, even though that's counter-intuitive for algae raising.
Gracilaria's slow growth rate under low light, and its dense structure, make it a rich source of very complex carbs and vegetable proteins, which makes it an ideal food for Tangs and explains why IPSF cultures 'pods on it. Unfortunately, it grows so slowly that to use it as Tang food would require a large, dedicated, low-light grow-out tub - probably under just one or two Actinic NO tubes. Otherwise, if you try to just grow it at the bottom of the main tank, the Tangs would eat it all in very short order.