The present invention discloses a gain amplifier, capable of concurrently operating at three different frequency bands. The gain amplifier comprises an amplification stage for amplifying a signal applied to an input of the amplifier and a triple-band resonance load connected between a DC bias voltage and a DC bias input of the amplification stage. The triple-band resonance load uses a set of reactive elements to provide match in a first frequency band, a second frequency band and in a third frequency band, such as at 2.4 GHz, 5.8 GHz and 9.0 GHz. According to the gain amplifier of the present invention, it can effectively provide triple-band signal amplification and in-band interference suppression for various multi-standard coexist communication systems.