PvP Fending off 2-gate-Zealot?

I would say it really depends on ur opening and when do you scout it. I will assume that you opened 1 gate gas-core (with or without zeal before 2nd pylon)

Scenario 1- You scout his 2 gates on his base at first attempr or early enough that a Zeal didn't pop out yet. Just build a zeal at your base, when he gets there you should have 1 zeal of your own and goon incoming or out already (depending on map/positions). With pure micro u should be fine, DONT lose your zealot, since 2 more are coming your way. If he gets GAS while this is going on, the attack won't last, just defend and be ready to tech to gain advantage (You should be able to hold with 1 gate, you can add a second if u don't think u can manage), if there is no Gas, he is either pushing hard, proxying more gates or taking an exp behind it. Scout is key to know if he exps or not, if he doesn't exp, just be ready to add gates. DON'T PANIC!

Scenario 2- You see his zeal halfway to your base, if you have a zeal good, just add a goon and more zeals if needed, repeat 1, if you don't have a zeal and ur goon is halfway, add a gate and after the first goon add zealots if you can't do enough dmg with your initial zeal goon, pull probes if needed, kite with your goon as much as you can. Important DON'T PANIC!

Scenario 3- You didn't scout properly, you see 3 zeals coming your way, you have 1 zeal and 1 goon almost done. You can add a shield battery, add a 2nd gate and build zeals after ur goon, pull probes to defend if needed, if you can hold the ramp better, else retreat near ur gates/mineral, pay attention to newly spawned units so that they don't get trapped by his zeals and above everything DON'T PANIC!

Scenario 4- he is proxy 2 gating... Quit workers from gas, stop probe production, build a 2nd gate, add a shieldbattery and build zeals pull probes when ur zeas engage... and DON'T PANIC!

As for the cannon... is not an appropiate response, it demands 300minerals for something that doesn't help you gain back map control, if I was the other P I would just expand instead of continue the aggression if I see you building a cannon to defend.

Those would be my advices, I know it is easier said than done, I myself tend to go out of control when getting 2 gate'd, but after some practice you can usually fend it off, 1 gate gas is far superior, I consider 2 gate only a good "surprise" bo if unexpected, and ofc viable in some maps without ramp or in some positions due to easy fast expand.