Go to the Control Panel/Hardware and Sound and finally the Sound Panel. In the Playback panel your speakers should be set as Default. Click on the speaker icon to highlight it and then click on the CONFIGURE button in the lower left of the Panel. Make sure it's set for "Stereo" and then click the TEST and see if both speakers work correctly.
If they do not work correctly, try a headset connected to the headphone jack and see if you get both channels OK. If you do, I would suspect there is an internal connection problem with one of the speakers. If it's still in warranty contact Gateway to get it corrected.