Your Code should be like this:
<proxy xmlns="http://ws.apache.org/ns/synapse" name="XSLTTRANSPROXY_BRYN" transports="https,http" statistics="disable" trace="disable" startOnLoad="true">
<target>
<inSequence>
<xslt key="gov:/Complete.xsl">
<property xmlns:ns="http://org.apache.synapse/xsd" name="GUID" expression="fn:substring-after(get-property('MessageID'), 'urn:uuid:')"/>
</xslt>
<property name="messageType" value="application/json" scope="axis2"/>
<log level="full"/>
<send>
<endpoint>
<address uri="http://localhost:54150/"/>
</endpoint>
</send>
</inSequence>
<outSequence>
<send/>
</outSequence>
</target>
<description></description>
</proxy>
And you need to enable the tags mentioned in this document:- http://docs.wso2.org/wiki/display/ESB403/ESB+and+JSON. If you are not getting data in json try the service in other browser