The only time that I've found where you 2D deadspike from far away is if you IB before hand and call out the DS as opposed to reacting to it. Gauntlet Hades on the other hand almost always has to be called out, you'll almost never be able to react to it fast enough, unless you're expecting it and you're ready to hit 4D. At least, that's what it's been like in my experience.
Also, I'm not sure if this has been brought up before, but I don't think that 6D is completely useless in this matchup. Granted nearly everyone of Ragna's normals will beat it clean and it doesn't net you great damage or anything, but I have found a couple of uses for it, albeit ballsy ones that I don't recommend you rely on.
In the corner, or midscreen occasionally, a Ragna will lay down some DS pressure. You can't always 2D over it, especially if they cancel the first hit of 5D into DS, so occasionally you are forced to block it. Generally you would try and instant block and then dash up for pressure or something. However, if you regular block it then the Ragna will most likely dash up for pressure and most likely will use either 5C or 5B since they are both fast and long range pokes. If the Ragna 5B's then you can 6D underneath it because of it's low profile properties, but if they 5C then you're screwed. Like I said, it's ballsy but I found it to be rather useful sometimes. Especially if you hit it often enough that the Ragna starts going for low hits because they're afraid of getting low profiled, then you can use this to your advantage and 2D over the low hits or just jump out of the pressure.
Slightly less practical is using 6D to go underneath Ragna's 6D. It's possible and I've done it a few times, but it's not a very safe option if you mess up the timing on 6D. Even less practical is using 6D to low profile under GH, 4D is a better answer in every way, but if you feel like being swag then you can 6D under GH. If you really want to mess with your opponent then you can do 6D on wakeup and try to low profile whatever they're mashing out. Really though, this is only for just messing around. It's incredibly high risk for relatively little reward.
Anyways, 6D... not completely useless against Ragna, just not always (or ever) the best option.
