Pass optional request query param in gatling

Question

I am using Gatling 3.6.1.
I would like to attach gatling param to the request in order to force getting the value NOT from the cache. I would like it to be configurable, in a way to attach it when I want to force thing not to use cache and in other times I would like to use cache so I wouldn't attach the param or at least the unique value to it.

This is what I tried:

http("ProductReferencesPDP")
      .get(path + "products/${product_code}/references?currentPage=1&pageSize=25&lang=de").queryParam("gatling", session => System.currentTimeMillis().toString)
      .check(jsonPath("$..numberOfPages").exists)

And it attaches the gatling param always. But how can I make it optional, meaning someting like this:

http("ProductReferencesPDP")
      .get(path + "products/${product_code}/references?currentPage=1&pageSize=25&lang=de").queryParam("gatling", if withCache "" else session => System.currentTimeMillis().toString)
      .check(jsonPath("$..numberOfPages").exists)

But it doesn't work like this. I cannot access the session in the if clause or in u custom function for that matter.
Thanks!