GD Power Search Pro for bbPress integrates into bbPress without any need for custom coding. That is true only if the original bbPress templates are used, or if they are modified, but still follow original bbPress templates layout, and include all the original filters and actions. Without that, Power Search plugin can’t hook into the templates and modify the output.
The bbPress search page is located on the ‘search’ page: www.example.com/forums/search/. This page shows the simple search form only, and when it is used it loads the results page. GD Power Search Pro plugin can modify this template to load the advanced search form. By default, you can still have a simple search form, and to load the advanced one, add a query argument like this: www.example.com/forums/search/?power-search. This will force load the advanced search form. Through plugin settings, you can replace the default form without the need for adding query argument.
And, by default, bbPress doesn’t show the search form on the search results page, but GD Power Search Pro plugin has the option to change that and include advanced search form in a collapsed/hidden state.