2. How can I take advantage of this kind of dropdowns in my custom component (front end).
In J3, I have used with JHtml::_('formbehavior.chosen', 'select'); and it takes care of the js and css. but i wonder if you have implemented a similar solution with joomla 2.5 & gantry.
Hugo, this option ONLY relates to the Gantry admin, and yes it is in regards to the 'fancy' dropdowns powered by chosen in Isis admin template. Basically we turn those off by default in the Gantry admin because they really slow down the loading of the page. We have a fair-few select boxes and chosen is JS powered and has to go through and manipulate every one.
In short, we are not limiting to changing the dropdowns in anything but the Gantry portion of the admin. You can use the chosen functionality in your own components with no interference from us