City on the Missouri River, founded in 1854 and known as the 'Gateway to the West' (5)

I believe the answer is:

omaha

(Other definitions for omaha that I've seen before include "Cotton fabric used for curtains and upholstery" , "US city; D-Day 16" , "- - Beach (WWII)" , "This was old Indian territory" , "American Indian" .)

I've seen this clue in The Guardian.

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