Simple: Oregon still becomes a state, but its borders abide by the extreme US claim, and it never loses any land before, during, or after annexation and statehood.
The biggest changes would obviously be in regard to maritime travel and warfare. Given that the Gargantuan is a _deep_ deepwater creature, I imagine it would ignore boats for much of human history, until around the early 1900s when big metal warships with their loud engines enter the picture. _Especially_ submarines.
Just for clarification, this creature is from the Subnautica games.