"Gank" comes from the phrase "gang kill" and refers to the practice of multiple players ganging up to kill a single player or NPC. For example, in GvG players sometimes try to gank the opposing side's Guild Lord in an attempt to achieve a fast victory. Along with this, "Gank" has multiple other meanings, one of these being when a party gets heavily outmatched in numbers while in a PvE environment or in a PvP environment. Gank is also a term which includes the practice of sending one or more players around a back path to attack the NPCs of one guild's base while the other remaining members attempt to distract the opposing team and keep them near a landmark or the center of the map. By killing off as many NPCs as possible, the gank gives its team a advantage,if the opposing team has to fall back,they will have less NPCs to fight off the opponents, an advantage which is often significant enough to give that guild the win. From this version of the term 'gank' comes the term 'anti-gank', which is simply a gank team designed to defend its base against, or kill, an opposing gank team.
In Heroes' Ascent, the term gank is used to describe the tactic by which one or two teams gang up against another team to prevent them from winning. A team might gank if they realize that they cannot win, and instead of resigning, go after one team to prevent them from winning. They could also gank by immediately seeking to take another team out of the battle. They may whisper members of the third team and agree to team up. There are several reasons a team might gank but most commonly it is because they do not like the players or tactics of the team they chose to gank. Another reason may be to help friends, allies, or guildies on the third team win.