how to modify the function of GaussianBlur with mask based on GPU?

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There is no need to modify it since an implementation based on CUDA is already available in OpenCV (remember to Google before post!). To be more precise, a lot of filters are already implemented in cv::cuda class in OpenCV. So you have to compile OpenCV library from source with CUDA enabled and then you can take a look HERE.

HERE you find the documentation regardind the createGaussianFilter() function while in this link you'll find the documentation regarding the GuassianBlur().

I don't know if they have changed the implementation but as far as I remember it doesn't work with kernel bigger than 32, but you can overcome to this just running the filter more than one times (it's equivalent to run it with a bigger kernel)

If you are into reserach, maybe THIS paper can be useful to you to understand why and what is done.

What I'm giving to you are the link to the OpenCV 3.4.5 documentation, find the right one for your OpenCV version.

Cheers and have fun!