A lot of people seem to have wildly different ideas on what "ganking" is. Tip: it's name came from the phrase" "gang killing". In it's purest form, it is simply a larger group attacking a smaller group that has essentially no chance to win. Most people think of say 5 on 1 or 20 on 5, etc, but it could even be 200 on 30. A skilled, organize "gang" of say 4 to 8 can take on larger groups, taking them one at a time, or when they are focused elsewhere (such as trying to siege a keep). I've seen videos of exactly that, and it's impressive.
Some people associate 1 or more high level players going into a low level enemy zone to kill players who have no chance. First, that's not "ganking", it's "griefing". Second, it's not possible in ESO.