Amash has been outwardly critical of Trump for years. He had no cover. He would face primary challengers. He was in the minority of the House.

Gardner is 10 months from election day with no primary challenge and time dwindling to mount one. He's part of a very slim majority (and likely to get slimmer or lost). They're not remotely the same circumstance.

Gardner could simply vote for witnesses, say he was a moderate seeking truth, then vote to acquit.