Teleport homers produce a signal that can be locked onto by teleporting troops. If the template used by the teleporting Grey Knights to make a Deep Strike is centred on the model with the homer, then they won't scatter. Note that the homer only works for units who are teleporting, not for ones entering play using jump packs, drop pods or other means of transport. Also note that the homer must be on the table at the start of the turn it is used.
Teleport Homer
Source: Codex: Daemon Hunters, p. 18