/usr/src/gcc-7/debian/watch is in gcc-7-source 7.3.0-16ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | version=2
ftp://gcc.gnu.org/pub/gcc/releases/gcc-(7\.[\d\.]*)/ \
gcc-([\d\.]+)\.tar\.xz debian uupdate
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