Starting with version 2.0.0, Gretty supports overriding versions of Jetty, Tomcat and Servlet API.

All you have to do is to add one of the following properties to "gradle.properties" of your project:

jetty7Version = 7.6.16.v20140903
jetty7ServletApiVersion = 2.5
jetty8Version = 8.1.8.v20121106
jetty8ServletApiVersion = 3.0.1
jetty9Version = 9.2.22.v20170606
jetty93Version = 9.3.20.v20170531
jetty94Version = 9.4.6.v20170531
jetty9ServletApiVersion = 3.1.0
tomcat7Version = 7.0.78
tomcat7ServletApiVersion = 3.0.1
tomcat8Version = 8.0.44
tomcat8ServletApiVersion = 3.1.0

Above we show default values of these properties. You can set them to other values, corresponding to existing artifact versions at jcenter or maven central.