Among sites using WordPress, Contact Form 7 is the second most popular plugin, right in between the two main SEO plugins: WordPress SEO by Yoast (#1) and All in One SEO Pack (#3). CF7 is found 6,41% of the times our tool WPThemeDetector analyses a WP site.

That at least is what our statistics say. You can see the Top 25 WordPress plugins here: http://www.wpthemedetector.com/top-wordpress-plugins/

You can´t see Gravity Forms appearing on our reports because it is currently #36, and we´re only displaying the top 25 plugins. I´ve just taken a look at our database to see where it was ranking.