Actually, if it is a strong enough hardware, you can run any GUI application that you'd like with vnc (like I do Ktorrent and aMule on my headless Arch server).
Here's how you do it:

1. Install basic xorg, a basic window manager and tight vnc on the headless server:

2. Run the server once so it will create ~/.vnc and setup passwords

3. Setup the vnc startup script (so you wont need to ssh and manually activate it after the headless reboots).
Call it vnc and put it in /etc/rc.d/
Dont forget to set it up to run at startup in rc.conf
replace daf666 with your username!

#!/bin/bash

# general config
. /etc/rc.conf
. /etc/rc.d/functions

case '$1' in
start)
stat_busy 'Starting vncserver'
su daf666 -c '/usr/bin/vncserver'
if [ $? -gt 0 ]; then
stat_fail
else
add_daemon vnc
stat_done
fi
;;
stop)
stat_busy 'Stopping vncserver'
killall Xvnc
if [ $? -gt 0 ]; then
stat_fail
else
rm_daemon vnc
stat_done
fi
;;
restart)
$0 stop
sleep 3
$0 start
;;
*)
echo 'usage: $0 {start|stop|restart}'
esac

QBittorrent is available as free, ad-less software licensed under GPL with source code available on GitHub.

Arma 3 Headless Client

Deluge SeedBox SeriesHowTo – Set up Deluge torrent server (aka seedbox) headless with UbuntuHowTo – Deluge torrent server (aka seedbox) Install XMPP Notify/Control plugin on UbuntuDeluge is a bit torrent protocol client, designed in client server model and has the ability to run in multiple and various interfaces. Client, web interface and console. Best Raspberry Pi Torrent Clients The Raspberry Pi makes a fantastic seedbox for an affordable torrent server. It's cheap but functional, boasts an incredible price to performance ratio, and with excellent Linux support, there are loads of different Raspberry Pi-compatible torrent clients.

This week, we’re looking at five of the best BitTorrent clients, based on your nominations. Which makes it ideal for servers, NAS boxes, HTPCs and other 'headless' systems. QBittorrent is one of the best torrent clients available that runs on the Raspberry Pi's ARM hardware. It sports both a desktop client and an easy to use web interface. One of the best parts of qBittorrent is its powerful web interface that allows you to control almost every aspect of the client with relative ease.

Arma 3 Headless Client Setup

4. To setup openbox to run at startup insert 'openbox' in your ~/.vnc/xstartup

5. Reboot the headless

Headless Server

Headless Windows Server

6. Install tightvnc on your desktop machine and connect to the headless: vncviewer 6.6.6.6:1 (replace with ur IP and screen num)

7. Install ktorrent or what ever.. then it will keep running after you disconnect