First of all, be aware that you're not allowed to customize Gatling's charting engine for your own needs, because of the Highcharts' licence:
https://github.com/excilys/gatling/wiki/License#wiki-highcharts
This said, you need your own charting engine/dashboard.
Regarding how to monitor your system, there's no open-source silver bullet that lets you monitor everything, you'll have to build your monitoring system on top of many components. For example, you can use JMXTrans to monitor JVM's and send to Graphite. This might look like something huge to build, but you should actually be using such tools for monitoring your live platform, and just reuse it for stress testing.