We report the discovery of a circumplanetary disk (CPD) candidate embedded in the circumstellar disk of the T Tauri star AS 209 at a radial distance of about 200 au (on-sky separation of 1.'' 4 from the star at a position angle of 161 degrees), isolated via (CO)-C-13 J = 2-1 emission. This is the first instance of CPD detection via gaseous emission capable of tracing the overall CPD mass. The CPD is spatially unresolved with a 117 x 82 mas beam and manifests as a point source in (CO)-C-13, indicating that its diameter is less than or similar to 14 au. The CPD is embedded within an annular gap in the circumstellar disk previously identified using (CO)-C-12 and near-infrared scattered-light observations and is associated with localized velocity perturbations in (CO)-C-12. The coincidence of these features suggests that they have a common origin: an embedded giant planet. We use the (CO)-C-13 intensity to constrain the CPD gas temperature and mass. We find that the CPD temperature is greater than or similar to 35 K, higher than the circumstellar disk temperature at the radial location of the CPD, 22 K, suggesting that heating sources localized to the CPD must be present. The CPD gas mass is greater than or similar to 0.095 M (Jup) similar or equal to 30 M (circle plus) adopting a standard (CO)-C-13 abundance. From the nondetection of millimeter continuum emission at the location of the CPD (3 sigma flux density less than or similar to 26.4 mu Jy), we infer that the CPD dust mass is less than or similar to 0.027 M (circle plus) similar or equal to 2.2 lunar masses, indicating a low dust-to-gas mass ratio of less than or similar to 9 x 10(-4). We discuss the formation mechanism of the CPD-hosting giant planet on a wide orbit in the framework of gravitational instability and pebble accretion.