What would normally be a boon becomes the opposite when this cursed ring is worn.  It will operate exactly like the 3rd level Transmutation spell, except it doesn't end.

While at first you may be excited, your enthusiasm will quickly wane once you realize that you can't find your finger to remove the ring.  You will remain in gaseous form until the proper magics can be cast.  The only way to remove this cursed ring is to be forced to return to solid form by a remove curse spell cast on your gaseous form (which will suppress the ring's effect for one turn) followed by a second remove curse spell before you return to gaseous form again.  A single wish spell, properly worded, will also suffice.

Source:  Dragon Magazine #77, 1983, page 20