I would take Gross where he is at in terms of development over Day. I've made it clear I was never that high on him (after watching him his D+1 year) but he did seem like a legitimate possibility for a bottom pair role down the road.
Day just seemed a step too slow, in terms of how he processed the game. And that's something that's really, really hard to overcome. He has interesting tools, but he reminded me of Michael St. Croix in that he just seemed to make decisions a split-second too slowly (MSC couldn't skate, but that's a different story).
In the end the Rangers absolutely could have kept both if they wanted to. They had another year to watch Day but determined they didn't want to invest any more player development time in him, which I can't fault them for. Gross, the reasons are less clear, but IMO have to do with his flattened development curve. I would have probably offered him an ELC but if they don't think there's much development room there, I get it.