Getting message at bottom of page:
Number of inputs on this page=2220
This form is very large and data will be truncated unless you edit php.ini. Add the line max_input_vars=5000 to your php.ini file on your server.
Unable to submit data.
Adam, this is to do with your server setup. Basically, your PHP is set up to not allow massive forms. You can ask your host to extend the value of max_input_vars in php.ini, and then this warning will go away.
This is actually not a bug, but a feature, as without this warning, if you submitted, lots of your permissions would be wiped out.
There is a post-install config guide on the project site (https://gibbonedu.org/support/administrators/installing-gibbon/). I did not have time to go through this after your install, but if you can find someone to pay for a couple hours who knows this stuff, it might be worth it.