The current supported version of Gatling as of the date of this article is version 3.2.1. A number of breaking changes were introduced as part of the 3.x release detailed here: https://gatling.io/docs/current/migration_guides/2.3-to-3.0/
Also, please check out our blog post about the differences between Gatling 2.3.1 and 3.0 from the perspective of someone trying to run those scripts on Flood.
If you don't yet have a Gatling script, here's a sample one that you can use as a test. You'll need to unzip it and upload just the
.scala file to Flood.