Lightly coat the inside of a 3-1/2- or 4-quart slow cooker with cooking spray; set aside. In a medium bowl stir together gingerbread mix and milk until moistened. Stir in raisins (batter will be thick). Spread batter evenly in the prepared cooker.
In a medium saucepan combine the water, brown sugar, and butter. Bring to boiling. Carefully pour brown sugar mixture over batter in cooker.
Cover and cook on high-heat setting for 2 hours (center may appear moist but will firm as it stands). Remove liner from cooker, if possible, or turn off cooker. Let stand, uncovered, for 45 minutes to cool slightly.
To serve, spoon warm pudding cake into dessert dishes. If desired, serve with ice cream.