Starting with version 1.0.0, Gretty supports unified interface to the servlet containers Jetty 7, 8 and 9, Tomcat 7 and 8. There’s only one plugin to include: org.akhikhl.gretty.
You can override these versions by properties, described in chapter overriding servlet container versions.
When you run generic tasks (appRun, appStart, …​), servletContainer property fully defines on which servlet container your web-app will run. However, you can use more specific tasks to override the chosen servlet container. For example, if you define: gretty.servletContainer = 'jetty9' and run task tomcatRun, your web-app will actually run on tomcat.
For more information see Gretty configuration and Gretty tasks.