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I understand that /gazebo is subscribing to the topic /parameter_events,

correct.

but what is rcl_interfaces/ParameterEvent?

It is the type of the topic. So the message type published to it -- and conversely: the message type a subscriber expects to receive on the topic.

I thought it was a node that was publishing to /parameter_events, but it doesn't show up when I run ros2 node list and it also doesn't appear as a node on rqt_graph.

it should be clear now why that is the case.

I also thought that it might be the location of a file, so I went to search in my ros2 workspace, but I didn't find anything with the same name.

Via some intermediate steps, rcl_interfaces/ParameterEvent is actually a file a package called rcl_interfaces. See index.ros.org/rcl_interfaces. The exact file is: ros2/rcl_interfaces/rcl_interfaces/msg/ParameterEvent.msg.

See Introduction to msg and srv interfaces for a (WIP) tutorial on how the message generation infrastructure is used.