What's the threat? An attacker who can observe only traffic on the wire (e.g. an ISP) probably gains nothing from observing GCM, since a) it's probably hard to distinguish different types of GCM messages and b) the attacker can just observe traffic to the Signal servers directly.

An attacker who cannot generally passively intercept (but not decrpyt) traffic on the wire but can passively intercept traffic to/from the GCM servers would be foiled by not using GCM, but that's a very narrow case.