for a Gtiff use GDAL

DEM's are normally a 16 bit signed image ( -32767 to + 32768 ) in 1 tone = 1 M
with sealevel set at ZERO

tiff dose not save to signed format( unsigned or float) you have to convert it to a 32 bit floatingpoint THEN save it as a tiff

for viewing them Nip2 works great and so dose the terminal version of Gmic ( custom source code build)