Hi Jason,
It appears that version of gtreamer was inadvertently deleted from the host site, hence the broken link in the WiKi. In fact, all the 1.9.x versions may have been deleted around 12/07/2020. Fortunately, you no longer need to download this package manually. According to the Mission Planner developer, @Michael_Oborne, it now gets installed when you install Mission Planner.
Cheers!