GObject itself is not a dependency, Inheriting from GLib.GObject provides support for features like introspection and better integration with the collections framework and GLib libraries. However, lightweight 'non-object classes' are supported but treated differently by the compiler. There is a slight variation in constructor syntax between plain objects and GObjects.
As far as I know there is no Vala runtime or garbage collector. Memory management is by automatic reference counting but there is support for manual C++-style pointer syntax with new and delete operators.
I agree that Vala was not specifically designed for 'low-level systems stuff' but I see no reason why this is not technically possible. The Vala compiler is a source-to-source translator to C and in my previous experience worked fairly well interfacing with fairly low-level C libraries like MPI and OpenCL. I assume that a kernel writer does not require much more than a limited subset of the language that supports C pointers, structs and arrays.