How can I use Gradle to just download JARs?

Something like the following should work:

apply plugin: 'base'

repositories {
    mavenCentral()
}

configurations {
    toCopy
}

dependencies {
    toCopy 'com.google.guava:guava:18.0'
    toCopy 'com.fasterxml.jackson.core:jackson-databind:2.4.3'
}

task download(type: Copy) {
    from configurations.toCopy 
    into 'lib'
}