Essentially, the PGtF is the 3.5 update for the Forgotten Realms. Not just the FRCS, but for more or less all 3.0 FR books. There's updated spells from Magic of Faerûn, for example. Beyond the necessary 3.5 updates, it revises some other mechanics (most noteworthy Home Regions), incorporates several D&D books into the Realms (Expanded Psionics, Manual of the Planes, Book of Vile Darkness, Book of Exalted Deeds), gives more detail on the FR-Cosmology, and has a small campaign journal, where some of the past events (As written in the first half of the War of the Spider Queen Series and in the Return of the Archwizards series) are explained.