What about a unit in a Himmelbett hex that was already deployed to a beacon (or a city)? Can it--after it has already deployed--still be marked with a GCI marker?
The second para lays out the conditions for having a GCI Intercept marker placed:
* The unit is in a Himmelbett hex.
* The hex is in the area of the unit's JD.
* No other unit in that hex is marked with a GCI Intercept marker.
* The unit is not marked with a scramble marker.
No other conditions restrict units so, yes, you could mark a unit that is already deployed.
However, the strictures of the 8th paragraph make it clear that deployment is immaterial. If a unit is deployed on a beacon or city then marked for GCI intercept, it could not infiltrate or conduct a wild boar attack. In essence, the benefits of deployment are null and void.
Note that the only way to leave GCI is to RTB, so there's no way a deployed unit could benefit from deployment if subsequently marked for GCI.