Last night, Reynold Xin took SPARK-3785 "Support off-loading computations to a GPU" off the Apache Spark road map, marking it "Closed" with a resolution of "Later". This is a little different than when GPU was mentioned at Spark Summit in June, 2015 as a possibility for Project Tungsten for 1.6 and beyond.
So for now, the best bet for using GPUs on Spark is Deeplearning4j, from which their architecture diagram above came. As I've blogged previously, the DL4J folks are waiting until they have solid benchmarks before advertising them. Nevertheless, today, you can do deep learning on GPU-powered Spark.