Can you please have a look if virtual/awk is sufficient or if gawk is needed.
docs/BUILD-NTOP.txt states "Some version of awk" and utils/*.awk looks very posix to me, so virtual/awk should be fine.
+ 10 Mar 2013; Christoph Junghans <ottxor@gentoo.org> ntop-4.0.1.ebuild, + ntop-4.0.3.ebuild, ntop-4.1.0.ebuild: + switch to virtual/awk (bug #455664)