A GKer (Generator Killer) is a survivor who destroys generators, often in an effort to grief other players, as part of a larger siege strategy, or even for no good reason at all.
GKing is sometimes considered one of the most effective Death Cultist or anti-Survivor tactics in terms of AP efficiency. The most efficient GKing weapon is a knife and the knife combat skill, which provides a 50% hit rate. As damage does not matter to a generator, which has five "hit points" equally damaged by any attack, this means that it takes an average of 10 AP to destroy a fully-repaired generator. In one 50-AP day, a GKer can move into an area, destroy 2-3 generators, and move out to safety with ease.
On the other side, for survivors, re-lighting a building requires a survivor to search for both a portable generator and a fuel can, and to install and fuel the generator, which (depending on location, search rates, and other factors) can run to 25 AP or more. There are also ancillary harms to the Survivor side, in the form of reduced search rates, inability to perform surgery in unlit hospitals, GKed mobile phone masts preventing phone usage, the risk of PKers and others hiding in dark buildings, etc. These difficulties, which are particularly aggravating to survivors, are considered another "benefit" for the GKer.
The only penalty for GKers is that GKing while a human does not grant XP (destroying a generator as a zombie grants 5 XP). Also note that beer and wine have a 100% hit rate on generators, but not a 100% damage frequency; damage rates from beer and wine have not been extensively studied.
Considering the impact that lack of light has on search rates — not to mention the fairly low search rates for generators — many zombie and anti-survivor players consider GKing to be one of the most effective ways of winning the "AP war" against the living. For this reason, some survivor groups keep track of GKers and hunt and kill them in much the same way bounty hunters hunt and kill PKers.