Wizkids Rules!

Loki Gemkeeper says

When fielded, choose an opponent's character card, canceling all previous choices. Your opponent cannot field that character while Loki is active. This effect lasts until you field another Loki.

I have Loki fielded and have chosen my opponents Hulk - Green Goliath.

My opponent uses the basic action Imprisoned to capture my Loki. Imprisoned reads

This die captues any number of opposing characters with a total fielding cost of two or less. This effect lasts until canceled or you damage your opponent (return the captured die to the field).

When I'm damaged and Loki returns, do I choose a new character card for his ability or is the choice of Hulk - Green Goliath reinstated?

The character returning to the Field Zone from somewhere else isn't considered to be fielding them. As a result, you won't get to repick for Gem Keeper. If you fielded a 2nd Gem Keeper while the 1st was Imprisoned, the 2nd choice would take over and cancel the previous one.

-The Dice Masters Rules Team