The old trailhead for the Gneiss Creek Trail was located at the river fording site at the Riverside Mail Station.
It was known by several names:
Riverside - Willow Park Trail
Gneiss Creek Trail
Gallatin Trail (though this seems like a misnomer)
This contrasts with the current Gneiss Creek Trail, which starts at 7-Mile Bridge, a few miles further down the road.
The trail was constructed in 1914 as a 27 mile long "fire lane" (at right, Report to the Secretary of the Interior, link).
This 1920 Yellowstone Rules and Regulations Map (at right - pg. 51: link) shows the trail leading from Riverside up to Obsidian Cliff.