There are certainly postings on both stack overflow as well as these forums that mention using cuda-gdb for inspecting memory.
I’m not aware of any that are specific to GpuMat objects. I’m not an OpenCV expert, and OpenCV is open source, so you should be able to deduce a method to symbolically inspect data in a GpuMat object. For a non-symbolic method, you could print out the value of the GpuMat data pointer and use that. Of course cuda-gdb isn’t going to know anything about data organization or padding within the buffer/object.
Unit 12 of this online training series discusses CUDA debugging with a section on usage of cuda-gdb. And of course there is the documentation.