With the release of Microsoft Dynamics GP in October 2019, it is moving to the Modern Lifecycle. This means the new version won't be called Microsoft Dynamics GP 2019 as thought, but will instead be the Microsoft Dynamics GP "October 2019" Release.
The Modern Policy offers continuous support and servicing, including bug fixes, new features and the latest tax updates. Customers can stay current by taking at least one of the three yearly planned Dynamics GP releases.
Before the October 2019 release, any older release will continue to follow their existing Lifecycle timelines under the Fixed Policy which you can see here.
Microsoft are committing to three updates per year with additional updates as needed (which would include changes like regulatory requirements such as the UK's Making Tax Digital).
You can see the planned update schedule for Microsoft Dynamics GP, beginning with the October 2019 release, on the Microsoft Docs website.
Microsoft MVP Jen Kuntz shared her thoughts on the change and I largely agree with her. Microsoft Dynamics GP is a mature product which doesn't undergo the large architectural changes a much less mature product, such as Dynamics 365 Business Central undergoes, as it already has the core functionality and the basic structure doesn't need to change as much. This means updates and upgrades are much simpler to undertake. A simpler upgrade once per year is a lot less work and a lot less disruptive than a major upgrade every two or three years.