This code uses the %seen hash idiom but resets the hash whenever a new host is reached; this might be tricky if host names are difficult to detect. It replaces empty lines by joining lines to be printed with newlines. Enclosing the selection code in a do block means the %seen hash is not left hanging around.

knoppix@Microknoppix:~$ perl -Mstrict -wE '
> open my $inFH, q{<}, \ <<EOD or die $!;
> hostname1.com
> 
> Gateway FAIL
> 
> Gateway FAIL
> 
> Gateway FAIL
> 
> hostname2.com
> 
> Gateway FAIL
> 
> Gateway FAIL
> 
> Gateway FAIL
> 
> EOD
> 
> print join qq{\n}, do {
>    my %seen;
>    grep {
>       %seen = () if m{^hostname};
>       ! m{^\s*$} && ! $seen{ $_ } ++;
>    }
>    <$inFH>;
>    }, q{};'
hostname1.com

Gateway FAIL

hostname2.com

Gateway FAIL

knoppix@Microknoppix:~$
[download]

I hope this is of interest.