Google will allow 3rd party developers to implement its Google Assistant anywhere, but not yet
The program that will allow other parties to make use of the Assistant is called Google Actions. Named after the simple fact that the assistant will divide requests into two categories — Direct Actions and Conversation Actions.
Direct is used for when the user's request is straightforward and gives enough information. For example “Hallway lights on”. A Conversation Action, on the other hand, is one that might require the Assistant (or the 3rd party service's own bot) to ask for some clarification. As an example, Google showed us how calling for an Uber ride via the Google Assistant would work. First, the user asks the Assistant for an Uber, then they are connected with the latter's own bot. The Uber bot will ask where you'd like to go and if you'd need a bigger car, for example.
The Google Assistant SDK for developers and businesses will be released at some point in 2017, and we will also be getting more information about Actions on Google this December.