It should be possible to add a GzipHandler in the jetty.xml configuration file.
This kind of handler (HandlerWrapper) doesn't look like to be supported: PaxWeb assumes that the first handler is of type JettyServerCollectionHandler.
I have attached a patch with allow me to configure this handler with the following xml fragment :
I've applied patch locally, configured etc/jetty.xml with your sample and checked that it works! I used mvn:org.ops4j.pax.web.samples/helloworld-wc/7.2.7-SNAPSHOT sample and Karaf 4.2.1.