We present Atacama Large Millimeter/submillimeter Array measurements of the ‘ Cosmic Seagull ’ , a strongly magnified galaxy at z = 2.7779 behind the Bullet Cluster . We report CO ( 3-2 ) and continuum 344 \mu m ( rest-frame ) data at one of the highest differential magnifications ever recorded at submillimeter wavelengths ( \mu up to \sim 50 ) , facilitating a characterization of the kinematics of a rotational curve in great detail ( at \sim 620 pc resolution in the source plane ) . We find no evidence for a decreasing rotation curve , from which we derive a dynamical mass of ( 6.3 \pm 0.7 ) \times 10 ^ { 10 } M _ { \odot } within r = 2.6 \pm 0.1 kpc . The discovery of a third , unpredicted , image provides key information for a future improvement of the lensing modeling of the Bullet Cluster and allows a measure of the stellar mass , 1.6 ^ { +1.9 } _ { -0.86 } \times 10 ^ { 10 } M _ { \odot } , unaffected by strong differential magnification . The baryonic mass is is expected to be dominated by the molecular gas content ( f _ { gas } \leq 80 \pm 20 % ) based on an M _ { H _ { 2 } } mass estimated from the difference between dynamical and stellar masses . The star formation rate is estimated via the spectral energy distribution ( SFR = 190 \pm 10 M _ { \odot } / yr ) , implying a molecular gas depletion time of 0.25 \pm 0.08 Gyr .