The study of brown dwarfs has progressed with astonishing rapidity in recent years. This science is described in detail in the Large Area Survey and the Galactic Clusters Survey but we note that similar science can be done with the GPS, detecting L dwarfs, T dwarfs and even cooler objects at very small distances. The strengths and capabilities of the GPS differ slightly however from those achievable by the LAS and GCS. There will be no Sloan i' or z' band coverage of the GPS, necessary to identify brown dwarfs via the steep rise in flux from 0.7-1.1 micron. Cool T dwarfs (such as Gl229b) can be confused with O-F type stars with similar near-IR colours. However, this will not be an issue near the galactic plane because an A0V star with T dwarf colours would have to lie at ~10 kpc distance to appear as faint as a T dwarf at 10 pc at J band. Such interlopers will readily be rejected by their reddening: AV > 10, E(J-K) > 1.7 mag. In addition proper motions detected by a few epochs of observation over several years will clearly identify sources at <100pc. The only sources which will be hard to identify will be objects at the L/T transition, with similar colours to M and L dwarfs, but the few sources in this narrow temperature range near 1400K will be adequately studied by the Large Area Survey.
The GPS will have the following strengths for brown dwarf work. (1) It will have high sensitivity at J band, to combat extinction toward distant stars, so it will be very sensitive to the coolest, nearest brown dwarfs. (2) The survey will be capable of detecting L dwarfs (11.8 < M(Jup) < 14.5, Kirkpatrick et al. 2000) to distances of several hundred pc. This sample of more distant Ldwarfs will allow us to confirm that the local space density is not unusual. (3) Detection of many brown dwarfs in star formation regions, as noted earlier. (4) Proper motion data which will provide an indication of source age. We expect kinematic differences between young L-type brown dwarfs and old L-type red dwarf stars which have cooled off the M-dwarf sequence after many Gyr.