A grey Gundamā¢--brand marker is about 99.9% identical to a 005 Sakura Pigma Micron, just "not black". IMHO, it's more like "transparent black" than "grey", if that makes sense. Useful at times, but I rarely use mine. (I also find the Sakura-brand stuff to be much easier to smudge/erase/blend/fix mistakes with--just how the ink behaves)
I might use it all on my 1/48 Milia though, as "black" would be far too stark, and I want a more subtle look on the red.