How about this:
add_filter( 'wp_generate_attachment_metadata', 'delete_fullsize_image' );
function delete_fullsize_image( $metadata )
{
$upload_dir = wp_upload_dir();
$full_image_path = trailingslashit( $upload_dir['basedir'] ) . $metadata['file'];
$deleted = unlink( $full_image_path );
return $metadata;
}
Not 100% sure everything will work ok without the main image, but it does the trick. You wont be able to regenerate the thumbnails / sizes as the main image is required for this.
Edit: I just re-read the question, I noticed you are wanting this to only happen when people upload via a form. Must be early for me, the above code will delete any attachment uploaded. I guess all you would need to do is find someway of checking if the upload was via your gravity form. Hope it helps you anyway.