Controlling diffracted waves has attracted extensive research interests these last years particularly for the potential application of beaming functionality. In this paper, we propose to realize on-axis beaming of diffracted electromagnetic waves by using a phase- gradient metasurface. The structure is optimally designed in order to transform surface waves to propagating waves and to enhance transmission through a subwavelength aperture. Both numerical simulations and near-field measurements are performed at microwave frequencies to validate the proposed concept. Furthermore, the metasurface is frequency-tunable and can be controlled by an external DC bias voltage. Consequently, by adjusting the electromagnetic response of each unit cell through the bias voltage, different phase gradients can be tailored, enabling broadband operation spanning from 9 GHz to 12 GHz.