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/​
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.