I would definitely side with NO here (Chapter B divider not withstanding). First, the pillbox rules specify:
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.
Secondly, the rules for Open Ground (B1.0) spell out what terrains are considered OG or modified OG (including hills, roads, shell holes...), gullies are not included. Moreover, the gully rules state:
19.3 Barring other terrain in the hex, a gully is considered Open Ground for TEM and Rout purposes, provided a LOS INTO it exists (see bridges; 6.32).
I would say that "TEM and rout purposes" means to exclude things like PB set up.
Finally, while I know that chapter dividers are generally assumed more complete than the RB, I do not see where the "Forifiable" column is even defined. Nor anywhere in the RB where the term is adequately defined. Am I missing something?