I think those pumps can fit on engines with more HP than a GX390, so it might be capable of 4000 PSI on another engine, but with a GX390 it will top out around 2500. Something like that.
Gear drive is better than direct drive. Direct drive, the crankshaft goes straight into the pump to turn it at the same speed… gear drive the crankshaft turns gears, which then work the pump at a different speed than the crankshaft is turning. Something like that. I think it’s better for the pump to be gear driven, and it pulls water from a buffer tank better. …something like that.