I am unaware of an automated process that can fool Captcha. This is probably coming from a real person.
You don't want to block all Gmail accounts... it's a very popular service with millions of perfectly legitimate users. Besides, this is NOT coming from Gmail anyway. The message itself is coming from your installation of TTX and it's being initiated by somebody sitting at a computer who can type any random address for the email. (you can enter
anything in the email box as long as it's formatted like an email address)
Just check your server log for the individual IP address and permanently block him from your site. If you're on Apache Linux, you can block an IP address with the htaccess file. You can do something similar with Windows Server but I cannot advise.
For the htaccess file in your www root...
order allow,deny
deny from 72.235.88.28
allow from all
That will block anyone at 72.135.88.28 from even being able to see your site.
Instead, using "72.135.88." will block all IP addresses from 72.135.88.0 to 72.135.88.255
For more than one, you can list them like this...
order allow,deny
deny from 72.235.88.28
deny from 75.158.78.16
allow from all