GFF files are the "Generic File Format" of Neverwinter Nights. A binary encoded file with a basic structure, it contains all the blueprints, area instanced information, even saved games contents.

Editors

The game comes with a very basic Windows only GFF editor, good for simple changes. In NWN:EE it is located in Neverwinter Nights\util\win32\GFFEditor.exe

There is the tool TlkEdit2 (2da/tlk/gff/erf Editor) on the vault, this runs in Java so is more cross compatible and may be easier to use.

Then you can decompile the files into JSON using the neverwinter-nim tooling. This is good for mass changes or uploading to a code repository.

Editing GFF Files

There may be references on other wiki pages to editing GFF files directly - the toolset either doesn't support specific changes (eg; higher numbers than the toolset shows) or doesn't support the changes full stop even if the game recognises them (eg; setting a NPC familiar type).

To edit the modules GFF files first open it so it extracts the contents to the Documents\Neverwinter Nights\modules\temp0 folder. Then find the file you wish to edit in your editor of choice (or copy it elsewhere, then copy it back). Once edited save the module and it'll package up the file without question.

Editing it later in the toolset may change the GFF fields back to some default or remove them entirely - so be warned changes made using a GFF editor may be undone at any moment.

Editing GFF with nwscript

You can convert most GFF formats to Json with ObjectToJson and back from Json to GFF formats with JsonToObject. Limitations here are the bigger fields like DWORD or DWORD64.

File Format Specifications

You can read most of the generic information in the GFF format specification Bioware made with a rundown of how it operates in the PDF.

Here is some cribbed information for ease of access.

Data Limits

This is the most useful information for many basic edits.