It's under the Financial Calculator, called Future Value (FV button). You enter the periodic payment (principal), interest rate and the period. Then press Calculate. I haven't been able to produce a correct result. But I might be doing something wrong.
More instructions here (scroll down): http://flsdemo.nrcfoss.au-kbc.org.in/content/docs/Gnome/gnome/users/gcalctool/5.28/gcalctool.html#gcalctool-financial-mode
