It's just a name and some fluff to make things more sensical from a story perspective. The TR-5 is a Gaplant with minor improvements and made more manageable so the pilot doesn't have to be a Yazan-tier guy or a Cyber Newtype to handle it, it's from Advance of Zeta. The Gaplant Kai (or Custom, really) is a stripped down version of the Gaplant that was first introduced in one of the SD Gundam G Generation games. Typically, it's stuck as a mobile armor but has better armaments than the severely underarmed original.

This is just an existing Gaplant that doesn't kill regular pilots, to avoid the wankery that'd come with putting my character on Yazan's level, with the Kai's weapons such the one that can be seen [url=http://i.imgur.com/sAAsYdb.jpg]here[/url], and you'll notice I've omitted the pair of beam guns on its back. It's not really a fanon machine. If you think about it from an upgrade progression it's Regular Gaplant-Gaplant Kai that isn't stripped down into a mobile armor-Hrairoo parts which are a canon upgrade to the TR-5.