/lib/modules/4.15.0-1018-gcp/build is in linux-headers-4.15.0-1018-gcp 4.15.0-1018.19~16.04.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.15.0-1018-gcp
/usr/src/linux-headers-4.15.0-1018-gcp