We report the detection of a gaseous circumstellar disk around a relatively old T Tauri star, GM Aurigae. Maps at 4″ resolution in λ = 1.4-mm continuum emission and in the 13CO (2 → 1) line reveal unresolved and compact dust and gas associated with the stellar position and at the core of a larger rotating gaseous disk, 950 × 530 AU in extent. The mean velocity gradient across the disk, which is oriented along PA ≈ 50°, is consistent with rotation about an axis at PA = 140°. The structure observed in 13CO aperture synthesis maps agrees well with synthetic maps of the gas emission generated from a model. For a disk that is inclined 30° from face on, in Keplerian rotation, we derive a 0.80M☉ central mass (star + disk), a systemic velocity, vhel , of 15.38 km sec-1, and a mass, 0.1M☉. The spectral energy distribution of the dust continuum emission suggests a very similar mass, 0.09M☉.