The Preferences → Play, Default HALion plug-in option was introduced in Dorico 4 to allow those who own the full version of HALion to use that in place of HALion Sonic SE 3 or, more recently, HALion Sonic 7.
Unless I am missing something, there is no Default Groove Agent plug-in option to allow those like me who own the full version of Groove Agent to use that with Dorico 5 in place of Groove Agent SE. Is this on the roadmap?