Ganking is fun. Being ganked several times without any possibility to escape isn't. But it's the risk I'm willing to take.
I rarely ever gank people just because of the fact I don't want to be ganked myself. The more you gank (lvl 90 ganks) the more enemies you'll make and the higher the chance you'll be ganked yourself.
Though, when somebody ganks me while killing mobs or ganks me several times in a row without a fair chance I put them on my Kill on Sight list (the addon "Spy" has this feature, GREAT addon if you want to gank/be warned. It automatically detects nearby enemy players. You should really get that addon if you're into ganking
) and gank them when I see them.
Also, I get more joy out of annoying people rather than ganking. Killing nearby mobs/CCing them alone is fun to see them panic.