Dont show user submitted contnet on frontpage

You should exclude the category that your users are publishing in from the query that puts posts on your blog page. It depends a lot on the theme your are using, but there are many ways to exclude one category from the loop that populates the blog page.

Here is some very generic information about how to exclude a category:
http://codex.wordpress.org/Template_Tags/get_posts
http://codex.wordpress.org/The_Loop#Exclude_Posts_From_Some_Category
http://codex.wordpress.org/Function_Reference/query_posts#Exclude_Categories_From_Your_Home_Page

There's even a plugin that may help:
http://wordpress.org/extend/plugins/advanced-category-excluder/

How you do it will really depend on the theme you are using. If you can post a link to your site or the name of the theme you are using, maybe someone familiar with the theme can point you in the right direction. This isn't really a Gravity Forms issue but a WordPress issue to exclude one category. The posts in that category just happen to be coming from entries on a Gravity Form.

"How To Exclude Posts Of A Certain Category In Thesis" (August 2009)
http://thesistutor.com/how-to-exclude-posts-of-a-certain-category-in-thesis/

I've never used Thesis to know if this works, but this is the general procedure to exclude a category of posts. You might have additional success getting help at the Thesis Forums:

http://diythemes.com/thesis/thesis-forums-open/