The gap that was solved by the Erie Canal. Building that canal had already been under discussion for 20 years before Lewis and Clark. There was the small matter of the Appalachian Mountains standing in the way, but if DeWitt Clinton thought those mountains could be conquered, why couldn’t mountains in the west be overcome, as well.
The Mackenzie River took Mackenzie to the Arctic Ocean, not the Pacific. There is a legend that he called it the “River Disappointment”
It was in his second expedition that he reached the Pacific Coast. He followed the Peace River, then by a series of portages and trips on the West Road River and down the Bella Coola that he reached tidewater on the Pacific.
I don’t think this is what @md-2000 meant by gap. In that post, the gap seems to refer to the Hudson River being some kind of gateway into the central USA, rather than a gap within the Hudson River that meant it could not serve as a navigable waterway.
Boy, was I off! I looked at some website to refresh my memory of things and it clearly stated that Mackenzie had reached the Pacific coast in 1789. I can’t find that site now, but it is clearly in error.