To use {5}
you need to enable re-interval
like this:
awk --re-interval '{ gsub(/.{5}/, "&\n" ) ; print}' file
awk -v WIDTH=5 --re-interval '{ gsub(".{"WIDTH"}", "&\n"); print }' file
You could also use --posix
too, but it will disable other functions in awk
awk -v WIDTH=5 --posix '{ gsub(".{"WIDTH"}", "&\n"); print }' file