The form is created from the shortcode which has the following structure (with all parameters):
[GK_Contact fields="email,name,subject" recaptcha="off" info="" email="" email_title="Contact Form"]
In the fields attribute you can specify which fields will be displayed.
The recaptcha parameter will enable reCAPTCHA when you set it to on and the reCAPTCHA public and private keys will be set under the plugin settings page.
The last two parameters are connected with the e-mail which will be send by the plugin. The email parameter specifies the recipient e-mail. If it set to blank string, then the page administrator e-mail will be used. The last parameter can be used for changing the default e-mail subject.