This procedure is based on a Debian installation of fail2ban, but it should works in any installation.
First, create a new filter file gitblit.conf in filter directory (Debian: /etc/fail2ban/filter.d/) or into filter.conf file. Here an example:
[Definition]
failregex = could not authenticate .*? \(/<HOST>:[0-9]*\) for SSH using the supplied password$
ignoreregex =
Then edit jail.conf to add "gitblit" service (Debian: /etc/fail2ban/jail.conf). For example:
[gitblit]
enabled = true
port = 22
protocol = tcp
filter = gitblit
logpath = /var/log/gitblit.log
Restart fail2ban to apply (Debian: /etc/init.d/fail2ban restart).