I didn't really want to get into a nitpicking geekfest over this, but if you insist...
Gandalf, in addition to being one of the magicians, is also a malar. The malar are the servants of the great gods, the valar, and are godly unto themselves.
It's maiar, the plural of maia, not malar.
Solars in the fluff text (somewhere... I forget where... maybe a 2nd edition book...) are described as beings that were powerful enough to become gods themselves, but instead chose to serve another god. They are the "champions" of the gods. To me, this means in D&D terms that the maiar are on the same power level as solars. Solars aren't officially given a DR in their 3.X D&D stats, hence why I said I think Gandalf in D&D terms is a
DR 0 Solar. If you look at the immunities, Spell Resistance, and other stats Solars are given in 3.X, you'll notice that they do have alot of the qualities of DR 0 beings, though.
His power is comparable to that of sauron without the ring.
Which doesn't really tell us much, because we don't have Sauron's D&D stats, either.
He clearly has some divine ranks, somewhere around 3-4
The maiar were not really full fledged "gods" themselves, but were "divine" ("godly"). They were servants of, and clearly lesser beings than, the valar (the "real gods"). At most, I think they'd be DR 0. Although, just being powerful outsiders would make them immortal. So even giving them that much of a DR is debateable, I think.
He is also the guardian of the ring of fire, one of the three elven rings.
Which also does nothing to help us determine what an equivalent being in D&D would be. It just means he has a magic (artifact, likely) ring.