How can I specify a category for a Gradle task?

You just need to set the group property of your task. Eg (from http://mrhaki.blogspot.co.uk/2012/06/gradle-goodness-adding-tasks-to.html)

task publish(type: Copy) {
    from "sources"
    into "output"
}

configure(publish) {   
    group = 'Publishing'
    description = 'Publish source code to output directory'
}