I'm thinking it is an action, and he stops attacking and just runs after doing the steal action, but he still has to follow the action-move or move-action rule. Otherwise, you get a situation where he can move up 10 spaces, steal and item, and move back 10 spaces in 1 turn. That seems too powerful. They need a chance to get him. Also assume he cannot open doors or anything--so when it is the end of the quest in first solo quest, he can't just run out the exit door. He would be cornered there. This is pretty unclear, and normally I don't have issue with making rulings and considering what I want it to be like, but also curious how others have ruled it?
"During Zargon's turn, each Ice Gremlin can either attack a Hero or Mercenary, or it can steal one item from one Hero (Zargon's choice). The item stolen cannot be the armor or shield a Hero is using, nor the weapon he is wielding. As soon as an Ice Gremlin has stolen an item, it runs away at full speed. The Heroes can chase the Ice Gremlin on their turn. If they catch it and destroy it, they regain the stolen item. If no Hero can see the Ice Gremlin at the start of Zargon's turn, the Ice Gremlin has escaped with the item. The item should be crossed off the Hero's Character Sheet. Remove the Ice Gremlin figure from the gameboard."
My group just started this quest pack tonight. We have played the others chronologically. The main change we have made other than this is to give an extra hp point to the Halberdier to differentiate him from the swordsman (hp tank-bruiser), and give him an extra movement point an extra defend die and allow 40g rehire cost for all Mercs as a hybrid rule from the UK expansions with the BQP body points.