for this simple build file:

usePlugin('java')
usePlugin('project-reports')

import org.gradle.api.tasks.diagnostics.GraphvizReportRenderer

repositories {
mavenCentral()
}

configurations {
checkstyle
}

dependencies {
checkstyle 'checkstyle:checkstyle:5.0'
}

dependencyReport {
renderer = new GraphvizReportRenderer()
}

run
gradle dependencyReport

you will get dependencies.txt (why not dependencies.dot ?) in build/reports/project

it looks like this (with SomeConf digraph repeated many times):

digraph SomeConf{
}
digraph SomeConf{
"commons-lang;runtime" -> "junit;master";

"checkstyle;default" -> "google_collections;compile";
"commons-cli;compile" -> "commons_lang;master";
...
"commons-beanutils-core;runtime" -> "commons_logging;runtime";
}
digraph SomeConf{
}
digraph SomeConf{
}
digraph SomeConf{
}
digraph SomeConf{
}
digraph SomeConf{
}

if you try to craete png out of it:
dot -Tpng build/reports/project/dependencies.txt -o graph.png

you will get complete garbage

if edit the dependencies.txt file by hand and remove all empty SomeConf digraphs then you can generate the png

tested with gradle 0.8