i have find theses point of rules :
for pillbox :
30.1 A pillbox is a Fortification counter that can be set up only prior to the start of play, and only in Open Ground (including any road/shellholes but not bridges), brush, orchard, grain or woods
for gully:
19.3 Barring other terrain in the hex, a gully is considered Open Ground for TEM and Rout purposes
i don't know if it's the good explanation but it seems good ...
