Configure path http.ssl.trustStore.file = "trustStore.jks" in gatling.conf

Yeah, the file parameter is currently expected to be an absolute path on the filesystem, but we could search the classpath as well.
Could you open an issue on Github please?
Note that we’ll only implement it in Gatling 2, we’ll soon enter Release Candidate phase for Gatling 2 and will stop maintaining Gatling 1 once final release is out.

Implemented, thanks to Ivan: