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 application is r.resamp.interp which can be used instead of r.mapcalc to peform bilinear and bicubic interpolation for the grid.