ちょっとやり方がわからなかったので。
GithubのIssueに書いてありました。
Create a 'between' query with ransack?
https://github.com/activerecord-hackery/ransack/issues/101
View
hoge.html.erb
<%= f.label :created_at_gteq %>
<%= f.text_field :created_at_gteq %>
<%= f.label :created_at_lteq %>
<%= f.text_field :created_at_lteq %>
Console
Item.search(:created_at_gteq => "2014-09-10", :created_at_lteq => "2014-09-25").result.to_sql
=> "SELECT `items`.*
FROM `items`
WHERE ((`items`.`created_at` >= '2014-09-10 00:00:00'
AND `items`.`created_at` <= '2014-09-25 00:00:00'))"