If so, have I to indicate anything in license (classical "About" window)?
1 Answer
Stock icons are taken from the user's currently used icon theme, you don't have to include them in your program. So you only have to follow the GTK+ license terms to use them.
If you want to ship a special icon theme with your program have a look at that icon theme's license if you are allowed to do so and what kind of attribution you have to show.