"Gift of the Wild" Reward:

Keep this card. You may discard this card after rolling a die to add up to 3 to the result.

"Misfortune" Spell:

Cast on any character who is about to roll a die. That die automatically rolls a “1” result and cannot be rerolled.

Can I use this reward after another player is casting this spell on me?

Yes. The Misfortune Spell states that the die automatically rolls a 1 which equates to still rolling the die. The 1 result cannot be rerolled but certain effects can add to the result.

Stuff that works at this moment:

http://community.fantasyflightgames.com/index.php?/topic/39174-misfortune-what-rerolling-really-means/page-2