Why do globe valves seal more efficently than gate valves?
I guess that depends on your definition of "efficient". Gate valves can provide a very tight shutoff, which gets tighter with increased pressure. For globe valves, it depends on the orientation. With flow from under the disc (the "normal" configuration for globe valves), increasing pressure results decreased sealing performance (ie, possibility of leakage). With flow from over the disc, increased pressure produces a tigher seal, but the valve can be hard to open. Globe valves, at least in my training, are not generally used as shut off valves, only for throttling.
*What to do if there's a high differential pressure?
With a gate valve, higher delta P means more torque is required to open. Use a gear (or electric) operator. As an alternative, use a bypass configuration: the torque required to open a small bypass valve is minimal, open that valve first and the let the pressure across the big gate valve equalize, then open the big gate valve.
With a globe valve, again it depends on orientation. With flow under the disc, tight shutoff requires high torque: again, think gear or electric actuator, or possibly hammer-blow handwheel. Opening is easy. For flow over the disc, closing is easy but opening requires more torque (actuator), or a bypass.
*Flow Curve

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