This means that your Wii U reset the connection from the TCP Socket. What I can suggest doing is trying again and see if it yields any new results - if nothing changes after a few tries try using a different version of TCP Gecko, I have a few in mind that I can provide. Secondly the newest version of JGecko is
very iffy when it comes to restrictive connections (which seems to be a constant when this error appears) Something I can suggest is switching to a slightly older version of JGecko; which I can also provide.
If none of these works then the only way to fix it would be one of two things, setting up your Wii U and PC on a Wired connection, as it could be a case of network instability. OR put your Wii U in a DMZ so there won't be as many connectivity restrictions. If you / your network admin doesn't feel comfortable with allowing a device to be in a DMZ (which is understandable) Then make sure this port is allowed to have traffic run through
TCP Port 7331
Another option would be to use the Python port of JGecko, I have it stored and documented here:
https://github.com/Korozin/JGecko_Remade-in_Python
Which brings up another troubleshooting thought - do any injectors / code scripts that require connection work? If so.. Does FTPiiU work as well? Because they all use some sort of TCP Connection - it may just be a case of JGecko U specifically not wanting to establish said connection.