Your choices are relatively limited, as the Gravelking SK is already one of the faster rolling gravel tires.
According to Bicycle Rolling Resistance, only the Conti Terra Speed, Michel Power Gravel and slick Panaracers roll faster.
it depends a bit on the size. The 48mm GK-SK is slow (and loud). From what I can tell the 35mm is pretty fast.
Similar with Schwalbe. BRR tests the G-One speed at 11 watts and the G-One ultrabite at 27 watts (these are the high pressure readings). Huge difference.
OP: In my experience, the G-One all round is about perfect for fast mixed useage. Its as fast as the "speed," I can't tell much difference between it and Conti's terraspeed, other than the Schwalbe wears much better.