I own the GS1K and HF-2, but I've never heard the other Grados, including the sr325i.
Based on my equipment and recordings, I'd say that the major difference between the two is synergy. The GS1K (w/ Black Dragon) is tough to match up. For me, the SQ is excellent only with the m902 via S/PDIF (coaxial or optical). With my other equipment, the bass is bloated with poor dynamics. With the m902, the entire spectrum, including the bass, is dynamic, warm, musical, fun. It's an SQ that I can't get with any of my other lineups. Thus, I can't let them go.
I haven't had the HF-2 for very very long, but the one thing I'm finding is that it synergizes well with all my equipment.
I tried the HF-2 bowls on the GS1K, and I found that the smaller pads actually made the GS1K sound very similar to the HF-2, but smoother and more refined with a slightly larger sound stage. And it seemed to work well with all my equipment.
When I received the replacement bagels (pads for the GS1K) and tried them on the HF-2, the result was imbalance, with boomy bass and a strange sound stage.
Based on this very informal and incomplete comparison, my guess is that you could emulate the HF-2 SQ with the GS1K using bowls -- but you can't emulate the GS1K SQ with the HF-2 using bagels.
My suggestion would be to get the HF-2 if possible. As an all-around, it's excellent.