Launch BGB and load a ROM, then go to File -> save memory dump...
Provide a start address of 8000 and length of 1800. This is where the tile data lives in VRAM:
gbromviewer tetris.tileset.dump
This should produce an output image matching BGB's VRAM viewer (ignoring the palette):
Tetris tileset viewed in BGB's VRAM viewer
Tetris tileset produced by gbromviewer (scaled 2x)
