As delightfully baroque as an early 80's Gamescience AD&D character sheet, Gamelords' Thieves' Guild products were one of the few series of third party products that accurately captured the tone of early AD&D: dark, mysterious, dangerous, and a little dirty. I always tried to grab their stuff whenever I spotted it - usually cheaply, as most gamestore owners didn't seem to really understand what it was, and would drop it into the "dollar box" with all the cheap traveller knock-offs and weird furry animal rpgs.
The rules, of course, were very similar to OD&D/AD&D, which makes these products very easily compatible with those systems. A glimpse at it, crossposted on the OD&D forums:
Thieves Guild occupies a space between being an AD&D hack and being its own game; it is obvious that the creators built it by expanding on the core AD&D framework, and there are innumerable artifacts of this heritage (e.g. the ability of thieves to use scrolls). The system occupies 33 pages in the initial TG installment, with the understanding that later products would fill out unexplored areas, which they did - if you buy an installment, it is going to be a grab bag of optional rules, modular encounters and small scenarios (usually around a common theme). For example, Installment 2 adds more weapon types (with adjusted damage ratings), combat rules, guidelines for hiring and maintaining hirelings, bandit and highwayman encounters, and The Tombs of Shale-Chuun scenario, which combines the mini-dungeons of Keep on the Borderlands with the traps of The Tomb of Horrors, and is a cool, flexible set of adventures. The approach to the rules is extremely modular, and it would be relatively easy to rebuild them to one's liking.
Ability scores include Strength, Dexterity (subdivided into Coordination and Reflexes), Stamina (also used to derive Magic Resistance), Intelligence (subdivided into Discretion and Talent) and Attractiveness (subdivided into Appearance and Magnetism). Abilities are rolled on 3d6.
Races are a mixture of tolkienesque and fairy tale: humans, elves, half-elves, dwarves, hobbits, kobolds, orcs, uruk hai, half-orcs, goblins, pixies and centaurs are all playable. Additionally, random social background, modified by race, is a part of the game, influencing starting wealth, as well as weapon and non-weapon skills.
Damage uses the contemporary 3rd party abbreviation HTK (Hits To Kill); however, HTK is not a function of class, but an average of Strength and Stamina further modified by a table based on the total of these two attributes (e.g. if the total is 10 or less, the character gains +1d4 HTK; if it is 10-19, it is +1d6 etc.). There are separate saving rolls, which are actually roll-under ability checks rolled with 2d12; they are also used for non-combat task resolution.
Combat: the game uses HAC0 (a precursor to AD&D's THAC0, which was listed in the DMG but left unexplained until later) to resolve hits; weapons have different HAC0 ratings, daggers being easier to use than maces or larger swords. Combat actions take place simultaneously, and there is no initiative, so a character who suffers a death blow may still strike back. There are critical hits and fumbles resolved with a relatively simple chart, and thieves are given additional combat maneouvres (backstabbing, coshing (KOs), poison use and striking from concealment. Armour works as in D&D, but the base value is 0 and goes up, so quilted cloth is AC 2, leather AC 4, chain AC 6, scale AC 8 and plate AC 10. Additionally, armour absorbs a small amount of damage (1-3).
Skills: there is a wide range of skills associated with social background; characters must spend double skill points to purchase skills above their social standing! The skills of Thieves Guild are much more socially oriented than D&D's, and include a loving attention to roguish pursuits (which are treated separately, so any social class can learn them without penalty).
Magic: a full magic system is absent from the initial installment, but subsequent modules add it gradually - in the scenarios of installment 1 and 2, there are example spell lists which work on a spell point basis, and are probably more mundane and less interesting than the exoticism AD&D occasionally went into.
Experience: experience is awarded for combat, the sale of stolen goods (very cool!), successful ability rolls and success during encounters (with success conditions set by the DM). Gaining levels results in the occasional extra hit dice (e.g. one is gained at 3rd, 5th, 7th, 10th and 12th level), hit probability (+2 per 4 levels), dodge bonus (to AC, +1 per 3 levels) and the increase of thief abilities. As a neat way to treat NPCs, Thieves Guild stat blocks classify them as Green (LVL 1-2), Intermediate (LVL 3-5), Veteran (LVL 6-7) and Elite (LVL 8+).
Further guidelines focus on fields thieves may find interesting - these are separate subsystems for disguise, fencing, traders, a treatise on guild membership and structure, a justice system (with a huuuuuge chart to determine outcomes; however, Judges Guild's similar guidelines are more colourful).
There are allusions in the books to parts of The Fantasy System that would be presumably released later (and might have been - I only own the first two installements) - classes such as Archers, Centurions, Healers, Mages, Priests, Priestly Mages, Illusionists, Assassins, Troubadours, Traders Hetearae etc. are mentioned but not detailed.
An NPC stat block for Thieves Guild may look like this:
In conclusion, it must be added that the scenarios and playing aids of Thieves Guild are done with obvious care and understanding, although they are not typical D&D fare - better suited to a game with more emphasis on social simulation and social encounters than improbable dungeon crawls. For example, the first installment has very detailed encounter modules with merchants, locales for cat-burglary (one set in the Street of Silk Veils, a red-light district; another in an area of magic and curio shops) and random encounters.
The production values of Thieves Guild are spartan; first printswere supposed to be three ring-bound, and were laid out on some primitive computer (most likely); reprints are in the shape of small booklets. However, they are also extremely dense with material and more material. Secondly, the artwork by Janet Trautvetter, if amateurish, is full of character and a human warmth that is missing from RPG art; her depiction of sensibly dressed fighting women is a particular plus.