Does it live in DISTDIR as a standard gzip'd tarball named
something like foozolix-1.2.tar.gz? If so, you can go on to the
next step. If not, you should look at overriding any of the DISTNAME, EXTRACT_CMD, EXTRACT_BEFORE_ARGS, EXTRACT_AFTER_ARGS,
EXTRACT_SUFX, or DISTFILES variables,
depending on how alien a format your port's distribution file is. (The most common case
is EXTRACT_SUFX=.tar.Z, when the tarball is condensed by
regular compress, not gzip.)
In the worst case, you can simply create your own do-extract target to override the default, though this should be
rarely, if ever, necessary.