You now have the ability to use Google Analytics Events in order to trigger reactions.
To set this up, start creating a reaction as usual - select a goal and a reaction type.
Next, when selecting a trigger, select the "Google Analytics Event" trigger:
An additional Settings view will appear, in which you need to enter the event information - Category, Action and Label (optional):
By default, the system will apply this reaction to all pages of the site.
If you'd like the reaction to appear on a specific page or group of pages, you can uncheck this or filter the pages - read more about global reactions here.