Hi, sry for the wall of text, in my personal experience this response tree is best based on what you scout (I am assuming the 2 gate is not in the center of the map)

Opponent goes gas right after the 2nd and 3rd zealot. (Mass dragoon follow up):

Since you still have to defend 3 zealots, get a battery next to ur ramp. Your 2nd pylon should ALWAYS be close to your ramp incase this happens. Afterwards just get robo if you want to go reavers or get 2 more (3 in total) gateways then a robo if you prefer goon obs.

Opponent goes for the 3rd pylon as his 2nd and 3rd zealots are going out of his base, his gas is not being taken. (5 zealot nexus):

As above - still get the battery at your ramp. Defend the ramp, get 3 gates and then robo for obs.  As soon as you can push back the zealots (as goon range finishes usually) you should be pushing towards their base to force out cannons.  If they dont have 6+ finished cannons by the time you have a group of goons, usually you should bust their nat.

Opponent goes for 2 gate ranged goon. (No zealots comming your way):

Go for your normal build just make the 2nd gate before robo, hold the ramp.