Faster RCNN training with matlab

I have been trying to implement a custom network with Faster RCNN in MATLAB, I have specified the ExecutionEnvironment as gpu in the training options for training both the RPN as well as the CNN for classification. But the RPN training is not utilising my GPU, and after hours of training on CPU it throws an error saying array (24GB) exceeded maximum array size preferences. I have a 6 GB NVIDIA 1060 GPU in my PC and it was working well with MATLAB for my custom classification CNN training. But when I include the faster RCNN (RPN) it is not utilising the GPU. Please help to resolve the issue