TeXmacs (http://www.texmacs.org) is a sophisticated word processor with special mathematical features. As well as being designed to nicely typeset mathematics, it can be used as a frontend for various mathematics programs, including giac.
Once you’ve started TeXmacs, you can interactively run giac within TeXmacs with the menu command Insert▸Session▸Giac. Once started, you can enter giac commands as you would in the command-line interface. You can later re-enter a giac entry line by choosing it with your arrow keys or clicking on it with a mouse. The TeXmacs interface also has a menu containing giac commands.
Within TeXmacs, you can combine giac commands and their output with ordinary text. To enter normal text within a giac session, use the menu item Focus▸Insert Text Field Above.