Which Republican Senator is the most likely to get blanched?

[Pessimistic Antineutrino]

If Darryl Glenn could come within 7 points of Bennet, then you're delusional if you think Gardner is doomed.

2016 was a Republican wave and Bennet isn't a particularly strong incumbent.

How is a year where the democrats gain seats in both houses of congress and win the popular vote a Republican wave?

Republicans won so many seats in 2014 that Democrats had nowhere to go but up. It was an R wave in the sense that Democrats didn't gain as many in either house than expected. The Senate is more about which seats are up, and which ones are open that anything else (see 2014, all those open seats in (Atlas) blue states made it look more R than it really was).

2016 was no more an R wave than 2012 was a D wave (at least they actually gained senate seats).
If anything it was a neutral year, with slight reversion to the mean on the Congressional level and a split on the presidential.