This paper designs and develops a Compton camera system suitable for nuclear decommissioning scenarios with a detector containing a 23×23 array of cerium-doped gadolinium aluminum gallium garnet [GAGG(Ce)] and row/column readout circuits. The detector is composed of an array of GAGG(Ce) coupled with silicon photomultipliers (Si-PMs). A subset-driven origin ensemble iteration (SOE) algorithm based on graphics processing unit (GPU) acceleration is developed to accelerate the reconstruction process. After development is completed, the detector performance and system performance are evaluated. In terms of detector performance, the energy resolution and position resolution of the developed camera are 6%±0.6% at 662 keV (full width at half maximum) and 2.2 mm, respectively. The detector also exhibits favorable energy linearity (goodness of linear fit R²=0.991). Regarding imaging performance, the initial imaging of a ¹³⁷Cs point source with an activity of 1.11×10⁷ Bq takes only 20 s when it is placed at 3.4 m in front of the imaging system. The angular resolution of the imaging system is 7°@JND (just noticeable difference). Finally, a simulation is carried out in the application scenario. The results show that the Compton camera developed in this paper can simultaneously image and position multiple radioactive sources within the π field of view. It can be used in radioactivity measurement in nuclear decommissioning to reduce the exposure dose of operators.