Cookie makes a good cite from case book, but it is still not listed as a rule, and "gorilla arm" was mentioned in a 2005 interpretation, term not seen in a rule or case book since.

The 2010 NFHS Baseball Rules Changes (B Elliot Hopkins, CAA) has 3 pages (18, 19, 20) clarifying the SET position with regard to the Gorilla Arm, and the word "Gorilla" is specifically mentioned on page 20:

"Rule 6-1-3
Set Position

• Umpires must be aware of the position of the
pitcher’s feet. There position determines if
the pitcher is in either the wind-up or set
position.
• The “Gorilla” stance is a legal stance of the
set position as long as the pitcher’s arm does
not swing. If his arm is swinging, this is
considered as a means to deceive the batter
and runner(s).

Rationale: Clarification."

I have never seen a batter or runner deceived by this in FED venues where it might not be strictly called. I have never seen a batter deceived by this in OBR where it is allowed. Someone might have an axe to grind in this case. If you look at the proposed FED rule changes that were voted on this year you might get a nuance of FED rule making?
BTW, I have seen gorilla arms but have never seen one swing. If you are in an area where they want it called then go ahead and call it. It's not a hard adjustment for the pitcher to not do it.

FED Never Ceases to Amaze Me...
 

From the NFHS Baseball Case Book:

"6.1.1 SITUATION J: With R1 on first base, F1, from the set position and prior to bringing his hands together while in contact with the pitcher's plate, (a) abruptly and quickly turns his shoulders toward first base in an attempt to drive back the runner; or (b) casually turns his shoulders to observe the runner at first base. RULING: Legal in both (a) and (b)."

Somebody please 'splain to me how the swinging gorilla arm is deceiving, and (a) is perfectly acceptable.

C'MON MAN!! :p