/usr/lib/linux/4.15.0-1027-gcp/compiler is in linux-buildinfo-4.15.0-1027-gcp 4.15.0-1027.28.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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GCC: (Ubuntu 7.3.0-16ubuntu3) 7.3.0