From what I remember of the series of books (I own them all), the cat's eye ring gave Gord 9 lives, the ability to shape shift into a small black cat or a large panther, gave him nightsight (infravision in the old system).
His magic weapons included:
The dagger that could cut through anything,he used it to cut through a treasure chest, and stone at one stage. It ignored armour in combat.
The shortsword that he carried gave him ultravision (1e sight that was a combination of low-light and darkvision - you saw in black and white but as far as normal at night). It was also able to give him a second saving throw (or similar) against certain mental attacks (the demon in the first book tried to lull him with magic and the sword saved him), and it also granted true seeing when drawn. Now it might have just given true seeing and the vision was part of the old spell (can't remember that far back).
He was a beggar/beggar-thief/thief/thief-acrobat in the 1st edition system (as stated by Gary himself) so his levels were something like thief/5 thief-acrobat/??
Since Gary had many optional extras in his campaigns (many of the dragon magazines he wrote articles regarding changes he used, one of the most outstanding in my mind was the one on Druids who fell from grace then redeemed themselves). So the beggar part of the class could have been and actual class or just flavour to gain skills before become an actual 1st level character (similar to the 0-level system that was in the Greyhawk hardcover book for 1e - was this based on some of Gary's work?).
He had ambidexterity and +1 to hit with sword and dagger.
The stats are fully defined in Saga of the Old City.
He was also highly educated (by a private tutor) in history, astrology, religion, etc.
In 3.5 this would be a simple case of a human rogue with all the traditional roguish skills as high as possible (pick pockets, move silently, hide in shadows, climb walls, use magic device, decipher script ...) and acrobatics and tumbling obviously. The 1st edition thief-acrobat had his own skill tables so it would simply be a matter of making sure your 3.5 version covered his bases.
He obviously has a good deal of knowledges under his belt, perhaps using some kind of feat to make knowledges class skills. Or you could possibly make him a bard/1 to just get them as class skills but disallow any spells.
The combat abilities could be described in terms of TWF and weapon focus in the appropriate abilities OR you could take the +1 to mean that it what is left over after factoring in TWF penalties. So perhaps he has Weapon Finesse instead, or both, no big deal there.
So let us see ... Feat for knowledges, TWF, Weapon Focus (Dagger), Weapon Focus (Shortword), Weapon Finesse
This would make him a Rogue 9 ... get rid of the Weapon Finesse and we have a Rogue 6.