I'm with JokeStroke on this. As a DS main and a Grundy second, this is one matchup I always dread getting as Grundy. If DS gets you full screen (not hard if he just lands the first hit really), and you don't have defense chain, there's not a lot Grundy can really do. Swamp hands get blown up by high or low guns, which come out quicker. WCC is his best option, but DS can just back jump or back dash to reacquire the space. Eventually if you're patient DS will have to corner himself, but that usually puts him on a corner interact, which will either let him damage and regain his spacing, or will vault him out of the corner, where he only needs one more quick knockdown to set up full screen again.
Grundy can also get in by burning super, but MB quick fire will do enough pushback to burn off a lot of the timer and DS can get away more often than not if he's conserving meter.
If Grundy does have defense chain it's still not free for him. He can turtle with life lead, but if he gets content to block DS can pop a trait and often block swamp hands by the time the Grundy player reacts, at which point the life lead will often vanish.
A Grundy victory is dependent on getting the first hit (or at least not eating a hard knockdown), activating defense chain, and keeping a trigger finger on swamp hands if he gets pushed back out afterward. A DS victory is dependent on knocking Grundy down, backdashing, and lolguns. Grundy clearly has a LOT more work to do in this matchup, and I wouldn't hesitate to call it 7-3 Stroke.