Where is the gmapping.cpp located in an installation from deb packages?

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Hello i have a relatively noobish question.

I want to change some things in the gmapping.cpp file so i can get the probabilistic values of the OGM but i cant locate the file in my machine.I installed Ros through Debian packages. The thing is that I am using gmapping and i don't understand where the file could be located. I searched in the /opt/ros/melodic/share-inc-lib and i couldn't find a file that handles the change from the probabilistic values to the standard ones.

Thanks in advance.