Consider the instructions on how to use Gerrit core's Gatling framework, to run non-core test scenarios such as this plugin one below:
$ sbt "gatling:testOnly com.googlesource.gerrit.plugins.renameproject.scenarios.RenameProject"
Scenario scala source files and their companion json resource ones are stored under the usual src/test directories. That structure follows the scala package one from the scenario classes. The core framework expects such a directory structure for both the scala and resources (json data) files.