Garritan Instruments for Finale (GIFF) is a library of high quality instrument samples that comes free with the purchase of a Finale license/upgrade/trade-up (for more details, see: Garritan Instruments for Finale (GIFF) vs. specialized Garritan libraries). If you installed and authorized the free trial or you customized your installation, you may not have GIFF installed even if you have Finale up and running. Here's how to check whether GIFF is installed so that you can use it with Finale.
Close the Launch Window if it appears. (You can also have any file open).
From the menu at the top of the screen, select MIDI/Audio > Sound Map Priority.
If "Garritan Instruments for Finale" is listed, then you have GIFF. If not, then GIFF needs to be installed.
If you do not have GIFF and you want to install it, see this guide: Reinstalling the Garritan Instruments for Finale (GIFF) sample library
Once you know you have GIFF installed, here's how to use it: How to use the Garritan Instruments For Finale (GIFF) sounds in Finale