Season 2: Till Death

Gabe takes it a step further, pulling Sam aside in the library and
confronting him with everything he knows. Gabe makes his threat to Sam (“You
better confess, or I am gonna tell everyone the truth about you. Starting with
Dana"
). Sam doesn’t say anything back but has a “evil ” expression
plastered on his face, so Gabe leaves. Sam detonates the bomb using the trigger
hidden in the box for his wedding ring.

I thinked that Gabe and Sam known one another and Gabe known something
about Sam because his threat seemed a speech  in code.