There are many solutions to avoid spam filling your gravity form with unsollicited trash messages. You can use the original gravity forms captcha field but I would not recommend this basic solution because it requires you to create an account on google's famous recaptcha service. Another drawback of the gravity forms captcha field is that it disables the AJAX option (read on gravityhelp) : if you want to use AJAX option or would like to have you captcha feature hosted on your server, you can use the famous wordpress plugin Really Simple CAPTCHA.
A third solution is to use simple rules on your form like asking the result of a simple mathematic operation like "what does 2+3 make" and use Min and Max on the number field type. Ok this is for reducing SPAM entries. But once you have caught thousand of bad entries in your form, how do you remove them quickly ?
I encountered the problem with a few thousand of SPAM entries dropped to my favourite contact form in less than an hour. Manual deletion of thousands of messages is easy but tedious when the default 20 entries per page makes you repeat the same operation a few dozen times. Here comes the gform_entry_page_size filter, detailed on gravityforms documentation : simply code the following filter in your thems functions.php file and depending on your server capacity you can substantially increase the number of entries displayed on the messages page, and accelerate the stupid deletion task..
And next time remember to setup antispam strategy..