A gain control method which includes setting a first initial gain value to a first variable gain amplifier; measuring a first power value corresponding to incoming signals; measuring a second power value corresponding to a target signal; and resetting the first initial gain value according to the first power value and the second power value. Another gain control method is also disclosed, which includes updating a gain value of a first variable gain amplifier by combining an adjustment value with the gain value according to a first tuning direction; obtaining a signal quality indicator; comparing the signal quality indicator with a reference signal quality indicator to generate a comparison result; and referring to the comparison result, further updating the gain value according to the first tuning direction or a second tuning direction opposite to the first tuning direction.