The tour group of seven people five Kenyans, a local tourist guide and a "non-resident foreigner" had been caught in the flash flood on Sunday in the park.
"Six bodies of the flash flood victims have been recovered, leaving one tourist missing," Kenya Wildlife Service (KWS) tweeted.
"The search and rescue operation continues as we reach out to next of kin to share details of sad incident and plan together next course of action."
Search and rescue teams had initially recovered two bodies on Sunday but four more were found as efforts continued throughout the night, KWS said.
A similar incident in April 2012 in the same park killed seven people who drowned in strong floods. The disaster led KWS to create emergency exists.
"Since the last similar tragedy in 2012, we have created clearly marked emergency exits along the whole gorge as escape routes in case of danger like the flash floods," KWS added.
"Every group is usually accompanied by experienced guides who are able to alert tourists of impending emergencies and direct them to exit points."
Hell's Gate is a popular hiking and cycling spot for tourists and locals and is known for its towering cliffs and gorges carved by a prehistoric lake. It inspired this year's remake of Disney's animated 1994 "The Lion King" movie.
( With inputs from IANS )