For nebulous, unknown reasons, the number six (6) seems to play a major role in the GPF universe. Nearly every number found in in the comic can be expressed as a multiple or power of six.
Perhaps the most famous example is the numbering of U.G.A. agents. U.G.A. agents are typically assigned a number that is a multiple or power of six. The reason for this numbering scheme has yet to be revealed. In general, the lower the agent's number, the higher their apparent authority or rank within the organization. For example, Agent #6 has a higher authority than Agent #12, who in turn has a higher authority than Agent #18.
Hotel room and apartment numbers often appear to be multiples of six. Here are a few examples from "The Safe-Dropping Incident", "The Honeymooners", and "Masquerade".
As Fooker keeps score while Ki and Trudy fight during "The Black-Tie Gala", he announces that "the current score is Trudy 6, Ki 5..."
"Miguel", one of Fooker's prison "buds", supposedly "robbed 6 banks in 3 days".
The airline boarding gate from the April 13, 2004 strip is Gate 24, a multiple of six.
In the "Negaverse", the Greys' holding cell was 5760F which, ignoring the trailing "F", is divisible by six. ("F" is, however, the sixth letter of the English alphabet.)
The Greys' hibernation cycle consists of a six-hour "shutdown" every 96 hours.
The code Pi gives to "Nega-Nick's" computer is "emergency revival sequence V6L42". The six is obvious, as is 42 which is divisible by six. "L" also happens to be the twelfth letter of the alphabet. ("V" is the 22nd letter, which is not divisible by six.)
GPF's booth at the trade show where Dexter had his heart attack was #1296.
The numerical designations of Grey alien drones do not appear to be multiples of six, but rather appear to be based on mathematical or physical constants. For example, Drone #3141592 is similar to the mathematical constant π while Drone #6626068 is similar to Planck's constant, at least when expressed in the standard SI designation of joule seconds.
One counter example to the hotel room numbering is the May 9, 2004 strip, which shows the number 602. However, it could be possible that Fooker and Maddie O'Hara share room 600, which is divisible by six. Since we never see their room number, however, this may be only speculation.