It is not a Gatling specificity. It has to do with how the JVM loads resources.
In real runtime environment, it is inside a jar (a zip formatted file for the JVM).
This specific file will be read-only.
However, you can copy it where you want (ie a temporary file for example) and modify the copy and use that copy afterward.
But, again, keep in mind that is not a gatling specificity and you should look for resources usage in the JVM (either java or scala)