This Tasker plugin lets you use the "Google Assistant" gesture (that by default fires up Google Now) as an event in Tasker to execute the task of your choice.
The gesture exists since Android 4.1 and can be activated by long-pressing on the Home button and then dragging to the icon that appears in a half-circle above.
It may also be available on some devices by hitting the Search button.
This plugin should also function in Locale and other apps that use the same API, but I have not tested with anything other than Tasker.
Note: This is not a full app on its own; it only functions in conjunction with Tasker or another app that implements the Locale plugin API.
Once you have installed this app, you will see the plugin as a context within Tasker (under State > Plugin). Then, if you perform the Assistant gesture, you should be asked which application to direct it to. Choose this application, and for best results, select that you want it to always be the choice.
Please contact me via email with any issues, as I can't respond to your review directly.
Neither this app nor Empty Path LLC are affiliated with Tasker, Locale or any similar app or author.