The chunk is 8 bytes long, and is of the format XXXXYYYY, where XXXX and YYYY are big-endian integers specifying the number of pixels from the left and top of the image, respectively.
This chunk must be present somewhere after IHDR and before any IDAT chunks, or ZDoom will not recognise it.
This chunk can be added to a PNG file from the command line using SetPNG or grabpng. SLumpEd and Slade 3 offer a graphical interface for editing it. DeuTex also has support for reading and writing grAb chunks when handling PNG files.
Outside of ZDoom and derived ports, the Doom 64 port Doom64 EX also uses this chunk.