User blog comment:YourPrivateNightmare/Hail the hourglass!/@comment-5331854-20130915215945

Let's compare Zhonyas with GA:

GA is 510 Gold cheaper, you have as much Armor as with Zhonyas plus 40 MR and while the passive has a high CD, it essentially gives you 130% maximum health in total after a delay of 4 seconds compared to 100% maximum health with a delay of 2.5 seconds. In most situations, GA as the defensive item is way more stat-efficient AND slot-efficient for ADCs than Zhonyas. It's also worthwhile when faced with hard CC.

While Zhonyas has its uses, it's too specific and niche for ADCs and GA will be a better choice approximately 99 out of 100 times. In fact, I can only imagine Zhonyas being better when the 1.5 extra seconds of GA would make a difference between "rest of enemy team gets to you" and "barely escapes before the gank occurs". And even then you pretty much have to foresee this specific situation happening.

If you have too much money, you can do the switcheroo and sell GA for Zhonyas after usage for double the invincibility, but it would be in a time where everyone's buying Elixiers and Oracles every time they call back AKA late endgame, so it'll be rare.