A system for filtering debris carried by stormwater flowing through a gutter inlet into a catch basin utilizes a filter device configured to conform to and to substantially cover the gutter inlet and a retainer device for maintaining the filter device in place covering the gutter inlet. The retainer device is attached to the filter device and configured to extend through the gutter inlet to hang gravitationally downwardly into the catch basin. The retainer device is weighted to gravitationally pull and hold the filter device securely against the gutter inlet against dislodgement under the forces of flowing stormwater.