Not sure even those involved in the final restoration know for sure today (perhaps they will chime in!), but in the following video, Gene Winfield describes building the mockup and also the interior set. He mentions the landing gear around time mark 6:20, though does not say what it came from specifically. I have also read that the rear gear may have been put together from pieces from more than one craft.
On another site dedicated to props, some people have done some great sleuthing to discover for example what "off the shelf" piece was used for the "chart recorder" used in the back of the Galileo interior set (as seen in "The Galileo Seven" for example). Again not sure it is known what the landing gear came from, but if not, I suppose it's possible it will be discovered one day.
I find it remarkable when people can piece this stuff together so many years after the fact, and after such "found" items used on the show have fallen into obscurity or are lost / nearly lost to history.