Model for classification Gender and Age with deepstream?

There is preprocessing in gst-nvinfer plugin. Gst-nvinfer — DeepStream 6.3 Release documentation. The gst-nvinfer plugin is open source, the source code is in /opt/nvidia/deepstream/deepstream/sources/gst-plugins/gst-nvinfer and /opt/nvidia/deepstream/deepstream/sources/libs/nvdsinfer. The diagram here DeepStream SDK FAQ - Intelligent Video Analytics / DeepStream SDK - NVIDIA Developer Forums can help you to read the code.