Gallic acid is a substance used to restore ancient documents. It can be found in Ker'aooc's house, given for free by his housekeeper, or bough in the shop of Port-Ludo for 50 kashes.
Once Twinsen gets the gallic acid, he gives it to his neighbour to restore a map belonging to his defunct son. The map shows Citadel Island's sewers and mentions a small Pyramid Shaped Key, which Twinsen's neighbour gives to him.
Inventory Description[]
"Gallic acid is used to restore ancient documents. It allows you to brighten up the areas where the ink has faded."