Before I give this answer I want to echo mxyplk's health warning on reading Gygax's prose. Suffice to say that much is kept deliberately (and confusingly) mysterious!
Gord, Gellor and Leda have three rings:
The three were forged in the the empyreal realms, and each bound a portion of Tharizdun's evil force into the Theoparts. The power of his nature is loosed now, but the Good which fought against it remains in our three rings.
They never really understand what these rings do:
Neither [Gellor nor Leda] was certain just what forces were contained in their respective rings, or how best to supply the dweomered energy to their comrade, but each did the utmost to act as a conduit.
The rings do allow planar travel:
"The rings will enable us to move quickly through the upper planes to reach the Sphere of Order.
And they also will make their auras awesome to all netherbeings. (I'm also not making this up!)
The rings seemed to provide some assistance in combat, but in what we think is the final combat with Tharizdun, Gord, Gellor and Leda appear nevertheless to be vanquished.
However, in the next (and, in fact, last) chapter we discover that in fact Entropy and Tharizdun fell into Chronos and Lady Tolerance's trap, somehow occasioned by the rings:
"Had the vile scum detected our trap, the whole would have comea cropper, as they say." "They sealed their own tomb, did't they, Chronos? And all to defeat a trio of mortals with little but some tokens of power?" "That they did, but the tokens were quite puissant, considering. Remember it was the three rings which drew all of Entropy's essence into that one pocket cosmos. It was if the entity willingly climbed into his own crypt and shut it fast afterward."
Tharizdun is not killed by the way - at some point he might perish, but Entropy will ultimately escape from the trap.