From an ATC perspective, what is an "arrival gate"?
Different ATC facilities need a standardized way to manage handoff of traffic between them and make sure traffic headed in different directions doesn't conflict at the border, especially at the interface between Approach/Departure and Center where traffic is climbing or descending and therefore can't be separated by cruise altitude like en route traffic can.
The standard way this is done in the US is to establish &quot;gates&quot;, which are fixes/waypoints on the airspace control boundary, and each is designated for arrivals (descending) or for departures (climbing).  Note that this boundary is not related to airspace class and is not charted, though you can often deduce where it is based on traffic patterns and SIDs/STARs.
Here is a conceptual diagram of the ATC gates for an airport in &quot;south flow&quot;, meaning aircraft use the runways in the southerly direction:

Source: here
The black crosses in the corners are the arrival gates, and the white crosses on the sides are the departure gates.  When the airport is in north flow, the diagram is flipped vertically.  For airports that are oriented east-west, the diagram will be rotated 90 degrees for east or west flow.  Most traffic will use a runway on the same side as its arrival gate, but there are crossover paths for when Approach needs to rebalance traffic or some gates are closed due to severe weather.
When a gate is &quot;open&quot;, there is an agreed maximum rate or minimum spacing that the sending facility can send aircraft through it, hence the term &quot;metering fix&quot; on the diagram legend.  If the sending facility exceeds this rate, the receiving facility may &quot;close the gate&quot; until they can sort out the resulting mess, which means the sending facility will have to slow down, reroute or hold traffic until the gate reopens.  Nobody wants that.