If 2020 is a 2008-esque blue tsunami, he's gone. If it's a neutral year, it could go either way. If Trump wins by his 2016 margins or expands on them, he wins.
Not really, no. Trump lost Colorado by a lot even as he won 304 electoral votes nationally. There is basically zero split-ticket voting happening in blue states anymore, and Trump has little chance of winning CO in 2020.
I'd say Gardner has the same "chance" of winning reelection as McCaskill does, honestly. Likely D, could be anything from a 2-point to a 9-point loss.