I have played with both. There are two GA8s, 47.5° and 50°, in two thicknesses, 2.0 and 2.2mm, and in two colors, red and black.
Red and black felt the same to me. The thickness differences had the predictable effect.
However, hardness did matter. At 50°, things clicked for me; at 47.5° it played, for me, much like a wide range of the (upper echelon of) ESN rubbers. I don't think I would be able to distinguish it from Vega X.
I prefer GA8/50 to T05, but still give T05 a distinct edge in spin creation. At the high end both were close, but below that T05 created more spin more easily. That advantage, for me, was more than compensated by GA8/50s handling in the short game, blocks, counters.