What do you classify as a gank?
The primary purpose of a gank imo is to relieve lane pressure. Not necessarily to get the kill, but to give your teammate breathing room in lane to try and use to get ahead. Sure, getting the kill is the best result. Getting a summoner is a pretty good one too. But simply punishing the enemy laner for pressuring yours is the primary goal of a gank.
That being said, anyone can gank pre-6. Some are just much more effective at getting early kills. Yi and Eve aren't the best pre-6, but you showing up and running off the enemy gives your laner plenty of space for a moment. Especially if you sweep wards and give them some paranoia of returning.
Wukong is someone I'm actually kind of surprised to see on your list - his early ganks are actually much more potent than they seem. Being able to clone in and jump people really only requires the coordination to have the laner either bait or lock down the enemy. You've got a gap closer and a fair amount of quick damage to deal and I'm pretty sure like 9.5/10 people will flash the actual moment someone comes out of stealth cause they panic. 6 makes them even more reliable since you finally get your own cc, but Wukong can get some straight Shaco levels of annoyance/pressure pre-6, just from being a filthy stealth champion.