The concept of "in good faith"

 

The things you did (the phone call to the old boss after the initial offer, the gift cards, the turning down of the 2nd offer) did not tell them you were in good faith.

The initial offer may have been just a negotiation tactic on the part of HR.  All companies want to hire with the lowest cost possible.  Your old boss may have taken your call as a counter offer.  You reneged, that made you appear not in good faith.

The appearance of "in good faith" is just as important as being in good faith.

A Thank You card is better because you can express yourself in words.  A gift card to a (potential) co-worker leaves too much to the imagination.

 

(The phrase "in bad faith" is never uttered, because it sounds really bad.)