It would probably be as disruptive as an at-will short-range teleport, which is basically what it is. Here's one way to write it up:
Portal Gun - Level 24 Unique Item
Price: 525,000 gp
Item Slot: Hands
Power (At-Will): Minor Action. Use this action to create one new Linked Portal, as the level 8 ritual, on any flat surface within 20 squares. The ritual is completed immediately, and requires no components. This action behaves as a ranged attack would (drawing Opportunity Attacks and requiring line of sight, for example). If two active portals created by this gun already exist, the wearer must choose one of them to deactivate as part of this action. A deactivated portal disappears permanently, leaving no trace. Any portal created by this gun will cause any creature activating it to teleport to the other active portal created by the Gun. These portals sometimes degrade as the wearer of the Gun moves in space, and may disappear without warning.