It works here, a copy of gambatte_sdl.6 is inside of the resulting package.
Code:
# make package
.....
===> Faking installation for gambatte-0.4.1
install -c -s -o root -g bin -m 555 (SNIP)/gambatte_sdl/gambatte_sdl (SNIP)/fake-i386/usr/local/bin
install -c -o root -g bin -m 444 (SNIP)/man/gambatte_sdl.6 (SNIP)/fake-i386/usr/local/man/man6
===> Building package for gambatte-0.4.1
Create /usr/ports/packages/i386/all/gambatte-0.4.1.tgz
Link to /usr/ports/packages/i386/ftp/gambatte-0.4.1.tgz
Link to /usr/ports/packages/i386/cdrom/gambatte-0.4.1.tgz
$ tar tvzf /usr/ports/packages/i386/all/gambatte-0.4.1.tgz
-r--r--r-- 1 root wheel 514 Sep 7 23:06 +CONTENTS
-r--r--r-- 1 root wheel 338 Sep 7 23:06 +DESC
-r-xr-xr-x 1 root bin 330652 Sep 7 23:06 bin/gambatte_sdl
-r--r--r-- 1 root bin 1937 Sep 7 23:06 man/man6/gambatte_sdl.6
$
Do you still have the man page listed in your PLIST file? it's still required.