gradle system test after jenkins deployment

I have a gradle project that is built by Jenkins within the following steps:

1. Build the project, run unit- and integrationtests
2. Build and publish docker images
3. Deploy application to pre-production
4. Run system tests on pre-production
5. Deploy application to production

Now step 4 is not ready yet and I am wondering about how to implement this in the most convenient way..?

I want to use JUnit5 and the tests should be inside the project itself, so either in one of the modules or with a separate module that is also included and has some other modules as dependencies.

When I create a separate module with normal JUnit5 tests for it, these Tests get automatically executed in Step 1. So what I want to achieve is a custom gradle step for these system tests. Any advice on this?