I just answered part of my question.
If you type in the wrong captcha, my form would redirect to send.php (current page - static url). But I needed it to redirect to a dynamic url such as send.php?gid=987
To accomplish this, I changed the the form's action from:
action="send.php?gid=<?php echo $row_rsGuns['id_inventory']; ?>"
Still working on other issues:
1. On page load, the validation message for the captcha shows right away "Please enter a value"
2. If you enter the captcha wrong it wipes out all entered form fields, so you have to start over.