The present invention refers to a gas burner (16), in particular a gas burner for a cooktop, comprising a plurality of burning portions (40, 42, 44). A gas burner according to the present invention comprises a plurality of burning portions (40, 42, 44) adapted to receive a gas flow from gas supply means (20, 30, 36) that comprise injector means (20) adapted to eject a gas flow diverging along the flow direction (10) to define a spreading angle Y. The gas burner is characterized in that gas-flow adjusting means (24, 25) are provided for modifying said spreading angle Y such that said gas flow is enabled to supply one or more of said plurality of burning portions (40,42, 44) depending on the adjustment of said spreading angle Y.