This page is dedicated to sparse and casual tricks with some common GFOSS tools.

This command works with relatively recent Gdal versions:

gdal_merge.py -o multiband.tif -separate band1.tif band2.tif band3.tif

Just use the builtin computing region facility, it will use a nearest neighbor interpolation for this purpose:

g.region rast=original_map -p
g.region res=<new>
r.mapcalc "new_map=original_map"

A similar trick can be used to cut a portion of the original map, apply a mask, etc. A more capable module for those applications is r.resamp.interp which can be used instead of r.mapcalc to peform also bilinear and bicubic interpolation for the assigned raster grid. A nearest neighbor interpolation can also be performed by the r.resample module as well.

See here for details about using GRASS with national service at PCN.

This is a very interesting thread about creating proj NTv2 precision grids starting from national precision grids in other formats, up to about 1:2000 scale and more.