It is possible to combine multiple raster files in one. It is useful for making masks. In short, we need to set the boundaries of working geographic region and then patch all maps together. Step by step:
Part I - g.region
1)Let`s set geographic region with g.region in command line and Run (GUI).
2)Choose maps which will define working region and only. Don`t choose anything else right now.
Part II - r.patch
1)Launch r.patch in command line, run with GUI it.
2)Choose raster maps (input)
3)Give name for resultant raster map (output)
Now you`ve got a new map in raster map list