In Gatling version 3.4.2, the typesafe.config library was updated, where, for security reasons, they removed the loading of all variables from ENV into the config. To override a variable from simulation.conf you need:
By setting the JVM property -Dconfig.override_with_env_vars=true;
Pass the variable in the CONFIG_FORCE_variablesName format, for example CONFIG_FORCE_intensity=100.