You could try the simple Perl approach to check if it works:

use Bio::Tools::GFF;

# $gff corresponds to your gff3 input file
my $gff_in = Bio::Tools::GFF->new(-file => $gff, -gff_version => 3);

# $gtf corresponds to your gtf output file
my $gtf_out = Bio::Tools::GFF->new(-file => $gtf, -gff_version => 2.5);

while( my $feature = $gff_in->next_feature) {
    $gtf_out->write_feature($feature);
}