I was looking for gcov source code that standard utility of gcc, I searched online but I was not able to find any URL or references for downloading Gcov source code ?
Apple uses GNU source (and BSD source), however it tends to have very old versions. If you search for...
gcov git repository
... you will find https://github.com/gcc-mirror, which says it is mirrored from git://gcc.gnu.org/git/gcc.git
If you want simply to browse the source, the mirror is suitable; for instance this file: