It looks like I may have found the problem, and it may be a bug in either SBT or the plugin.
Launching from the command line fails:
sbt gatling-it:testOnly com.example.testApp.ProveItWorks
Launching from the sbt shell works
sbt
…
gatling-it:testOnly com.example.testApp.ProveItWorks
If it helps to debug the issue, my code is checked in here: GitHub - JohnArrowwood/gatling-sbt-seed: Gatling starter project, with all my favorite helper classes