If you want a two-track (stereo) recording, following the main (AFL) mix of the event, you can use the 2-Track Output (see User Guide p.15) connected to a CD-R, Mini Disc, or MP3 recording device.
To record to multi-track, use whichever combination of groups, matrix, auxes or channel direct outputs is most suitable, for instance; by using the Matrix Outputs, the recording levels to a recorder can be can adjusted independently of the main mix, offering more control of your recording, or add ambience to the recording via a MIC connected to the Matrix EXT IN. The additional outputs from the Matrix also mean that if you're using a multi-track recorder, you can make more complex recordings of your events.
More details can be found on p.31 of the GL2800 User Guide. Appendix 4 of the Guide to Matrix Mixing also explains how the Matrix can be used for live recording.