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Metamaterials have been attracting tremendous interests due to their exotic electromagnetic characteristics, such as sub-wavelength imaging and invisibility cloak. However, the appreciable loss arising from the inherent resonant nature of metamaterials plagues many of their engineering applications. In this paper, we derive the condition for achieving gain media based on the conservation of energy, and propose a universal approach to achieve various gain metamaterials at microwave frequencies by incorporating active devices into traditional metamaterial building blocks. Theoretical homogenization models for such gain metamaterials are obtained from quasi-static considerations. Full-wave simulations show that microwave net-gain metamaterials with negative permittivity, negative permeability or even negative refractive index are all achievable. This approach can be used in applications whose performances are limited due to material losses.