Oh, absolutely you can change the logo. I think you misunderstood the addition to the GPL that applies.
Because the GPL generally applies to binary programs, and a large portion of Gargoyle are scripts and not statically linked C programs, it is unclear how to interpret the GPL. There are 2 parts to the addition:
1) If you use any of Gargoyle's code in your web interface your web interface must be GPL (with this addition/clarification)
2) You are permitted to improve/extend the web interface to configure proprietary back-end programs, so long as the additions to the web interface fall under the GPL. The back-end proprietary programs can remain proprietary.
So, feel free to swap in whatever image/theme you want. The only caveat is that code/image is GPL and you can't complain if someone takes that image and modifies it for their own purpose, also releasing it under the GPL.
On another note, I never thought of the Gargoyle image as being offensive. I suspect you're having issues with devout Christians who have issues with the resemblance to a devil, and yet the idea of a Gargoyle comes directly from Medieval (Christian) architecture. From
Wikipedia:
A local legend that sprang up around the name of St. Romanus ("Romain") (AD 631–641), the former chancellor of the Merovingian king Clotaire II who was made bishop of Rouen, relates how he delivered the country around Rouen from a monster called Gargouille or Goji, having the creature captured by the only volunteer, a condemned man. The gargoyle's grotesque form was said to scare off evil spirits so they were used for protection. . . .
Many medieval cathedrals included gargoyles and chimeras. The most famous examples are those of Notre Dame de Paris.
So, Gargoyles have a long tradition in the church as something meant to ward off evil spirits. The reason the name/image was chosen for the project is because Gargoyle is meant to ward off evil/unwanted packets on your network
