You probably need

my %gp = (
  ...
  Gi => \@gi, # an array ref
  Gt => \@gt,
);
[download]

Hash values can only be scalars. An array reference is a scalar so we can use that.

In your while loop you will first need to test if it is an array ref and, if so, dereference it. Something like

  ...
  $j = $gp{$i};
  my @values;
  if (ref $j eq 'ARRAY'){
    @values = @{$gp{$i}};
  }
  else {
    @values = ($j);
  }
  for $val (@values){
  ...
[download]

(not tested)
update: fixed typo