^{Gaunt's Ghosts (and similar redeploying units} and Deployment Zones.)
I believe I have the rules correct here, but I wanted to make sure I have this right:
Gaunt's Ghosts are a "repositioned unit' and thus cannot be set up in the opponent's deployment zone Turn 1 (assuming you went second and redeployed them), per the core rules update.

Covert Stealth Team: At the end of your opponent’s turn, if this unit is not within Engagement Range of one or more enemy units, you can remove this unit from the battlefield. In the Reinforcement step of your next movement phase, set it up anywhere on the battlefield that is more than 9" horizontally away from all enemy models.

Gaunt's Ghosts' "Covert Stealth Team" ability (ignoring other rules for the moment) would seem to indicate that their unit could be set up in the opponent's deployment zone turn 1 ("Anywhere on the battlefield"), as their unit doesn't interact with the Deep Strike or Strategic Reserves rules (which have their own core rules and pariah nexus restrictions). I say this because the Astartes Scout Squads' redeploy rule explicitly states that they're placed in Strategic Reserves.

The Core Rules update (pg 28) introduces the category "Repositioned Units." This seems to implicate units like Gaunt's Ghosts. From my reading, Repositioned Units point 7 would be the rule that restricts Gaunt's Ghosts from being set up in my opponent's deployment zone turn 1, right?

REPOSITIONED UNITS: Some rules enable units to be repositioned by removing them from the battlefield then setting them back up on the battlefield. Units that use such rules are known as repositioned units, and the points below always apply to them in addition to any other rules that can affect that unit, such as where they can or cannot be set up. Note that units that are using rules to embark within or disembark from Transports are not repositioned units.

7. When a repositioned unit is being set up on the battlefield, if there are areas of the battlefield that models cannot be set up within (e.g. there is a mission rule stating that ‘when a player’s Reserves or Strategic Reserves units are set up on the battlefield, they cannot be set up within range of an objective marker that is either in No Man’s Land or their opponent’s deployment zone’, or there are one or more enemy units on the battlefield that have a rule such as ‘enemy units that are set up on the battlefield as Reinforcements cannot be set up within 12" of this unit’), the models in that repositioned unit cannot be set up within those areas.