/usr/share/vala-0.16/vapi/gtk+-3.0.deps is in valac-0.16-vapi 0.16.1-2ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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gio-2.0 atk cairo gdk-pixbuf-2.0 gdk-3.0 pango x11