#!/usr/bin/perl
use warnings;
use strict;

my $previous_gn = "";
my $header;
while (<>) {
    if (my ($gn) = /^>.* GN=([^ ]+)/) {
        if ($gn ne $previous_gn) {
            $previous_gn = $gn;
            $header = $_;
        }
    } else {
        if ($header) {
            print $header, $_;
            undef $header;
        }
    }
}