Technically both Pinion Battle Demi-Company and Stormlance Battle Demi-Company (note the capitalization) are considered Core choices for the Gladius (at least they're called that in Army Builder). They contain the same restrictions as the Battle Demi-Company (requiring the same command in either a Captain or Chaplain) and most of the same composition rules (with minor differences). The problem -- to me, anyway -- as why there's even a question is that they're called XXX Battle Demi-Company. That's a pretty specific name. If it were named Stormlance Battle Formation there would be no question. But using that specific name makes me think you can use them.
In C:SM, the description of the Gladius Strike Force (the detachment in which you place Battle Demi-Companies in to create a Battle Company) says this:
"RESTRICTIONS:
This Detachment must include at least one Core choice and one Auxiliary choice. It may
include up to one more Core choice, up to three Command choices and any number of
additional Auxiliary choices, in any combination. Only the datasheets listed here can be
included in this Detachment. All units in the Detachment must have the Space Marines
Faction and must be drawn from the same Chapter." (Emphasis mine)
Now, this would seem to indicate the death-knell for my argument, however, we all know that this is true at the time of C:SM's publication. This does not mean that further rules could not (and more than likely would, considering GW's history) modify the restrictions on the GSF.
Personally, I'm of the opinion that unless FAQ'd, this one is open to interpretation.
NOW, because I like rulesy questions, I ask a new one:
In all of the new detachments where the Skyhammer Annihilation Force is allowed, it's an Aux choice. Does this mean it is also an Aux choice in the Gladius? Again, remember that Skyhammer was released only on the web previously. Unfortunately I don't know enough about how these things work to be able to make even an educated guess. Does anyone know if the original release included it as an Aux choice in GSF or not?
What say you?