You can't clone from Github on an OverTheWire (OTW) game server, but you can use scp to upload files.
On OTW server:
$ mktemp -d
/tmp/tmp.AaSsDdFf1
Locally:
$ git clone https://github.com/osusec/ctf.git
[output snipped]
$ tar cvzf ctf.tar.gz ctf/
[output snipped]
$ scp -P [game port] ctf.tar.gz [username]@[game].lab.overthewire.org:/tmp/tmp.AaSsDdFf1
This is a OverTheWire game server. More information on http://www.overthewire.org/wargames
[username]@[game].labs.overthewire.org's password:
ctf.tar.gz 100% 19KB 48.2KB/s 00:00
On OTW server:
$ cd /tmp/tmp.AaSsDdFf1
$ tar xvzf ctf.tar.gz
[output snipped]
You now can work with the CTF repo data on an OverTheWire game server, or any wargame system that blocks outbound connections!