Looks correct to me, except for the JSON serialization for which you should use jsonStringify(), but that wouldn’t explain the “No attribute” error.
Please provide a full reproducer as a project on Github.
#4226: The feed override that takes a number of entries now produces arrays instead of translating the keys with an index suffix.
So:
with Gatling <= 3.7 => indeed, you have planetNames1 and planetNames2
with Gatling >= 3.8 => you have a planetNames array whose entries you can access with #{planetNames(0)} and #{planetNames(1)}, or just directly use the array with #{planetNames}