Certainly in the next version of the release, there does not seem to be any plans to release GEdit specifics to the framework. I think the TextMate bundle was added as this was the development environment Guillaume uses when building his Play applications for clients, so its made sense to speed his development up.
There is no reason why this functionality could not be created outside of the Play core framework, as a module or plugin.
As for Niels question about the gedit://myfile:3, it does appear that GEdit supports opening a file to a specific line number, but I was unable to find a way to open GEdit from a registered URL (again like TextMate). This should be possible, however I am not a strong enough Linux developer to know how, but it would simply be a case of mapping the URL in a format such as
gedit://open?url=file://%s&line=%s
to the linux command
gedit +<line> <file>