Instead of the GLoG, I recommend these games

• BONEPUNK is a beautifully-formatted game in which you play the undead who are exploited in an industrial necromantic society. It is awesome in the "play a giant gorilla skeleton" way while still being tragic in a "sell your bone marrow" way. It is free here.
• G20 is only three pages long but it forms a flexible basis for many games. If you have four-level classes you want to use, and you want to play in ten minutes, this is a good way to do that. It is free here.
• The GROG combines light rules with the tactical combat of 3rd edition D&D, allowing for satisfying  combat play without sacrificing finesse-in-play at the table. It is free here.
• HEIMJING is a D&D-like game with a built-in setting that totally changes the way the game works (gravity is reversed for humans), even when you're just going into and out of a dungeon. Its zodiac mysteries offer an intriguing personal quest for player without disrupting whatever game the DM is running. It is free here.
• MARROW is a light, experimental classless system, where the weight of the system is in the management of different sorts of inventories not only for your belongings but also your skills, spells, languages, and more. Sort of like KNAVE to the max. It is free here.
• Runaway Princesses is a game made to be accessible to people who primarily have experience with the modern edition of D&D. It takes time to define its terms and give advice, all in a well-formatted document. It's not the shortest or lightest rulebook, but it can be familiar to people trying to branch out. It is free here. 
•  Vain the Sword is a mythic-folkloric fantasy game, a simplified hack of D&D-esque dungeon system. The rule set offers a tragically heroic skew on classic old-school problem-solving play, with stunning original art. It is free here.

There are plenty of other great games I could go into here, but the sheer number of free dungeon games online is an embarrassment of riches. If you seek more games, or inspiration for the great games you will write, please feel free to contact one of the community's servers here.