This would actually be pretty easy to do. It would require the Gate spell clickable, a defined cast time, a recast time, and be able to click from any slot. I believe these columns in the
items table should do it:
casttime: 10000 (10 second cast time)
clickeffect: 36 (spell id for Gate)
clicktype: 1 (should cast from any slot)
recastdelay: 1800 (30 minute cooldown)
recasttype: not sure (you'll probably need to find what other items use the gate spell, and use the same recasttype, unless you want it to be on its own timer)
Everything else should be able to keep the default values.