Possible to set new user’s site time zone at user creation using Gravity Forms?

Quoting from How to get WordPress Time Zone setting? you can set the option 'gmt_offset' with the appropriate offset from UTC.

You should add the option to control it only in you site creation form. If you put it also on user which registers to an existing site (like the main site) users will be able to change the setting on creation, something that only the admin should be able to do.

If your form calls wpmu_new_blog() you can set that option on blog creation from the data given in the form.

function wpse_127265_timezone( $blog_id )
{
    switch_to_blog( $blog_id );
    update_option('gmt_offset',$_POST['gmt_offset']);
    restore_current_blog();
}

add_action( 'wpmu_new_blog', 'wpse_127265_timezone' );

If that data is part of the registration form (meaning that the site is created only after the user activated it) then things get a little more complex and you need to store the data at some other place until the activation is done.