The realization of compact on-chip devices with spectroscopic functionality is one central goal in modern photonics. Here, we present a fiber-interfaced on-chip light cage that combines seamless single-mode fiber integration with extended light-gas interaction lengths, enabling precise quantitative gas spectroscopy. By employing two detection schemes, this work establishes the light cage as a platform for laser absorption spectroscopy, realized through 3D nano printing of an anti-resonant hollow-core waveguide with side access. Optical characterization and benchmarking confirm reliable gas detection at ppm-level detection sensitivity in a miniaturized format. The platform offers broad potential for applications in environmental monitoring, medical diagnostics, and quantum technologies, and provides a versatile foundation for the development of next-generation hollow-core architectures.