GIFF comes with Finale, and is built to work with Finale from the ground up.
GPO5 does not come with Finale. It's an Orchestral Library, sold separately of its own accord, and is meant to be fully compatible with a wide range of tracking DAW and Scoring applications.
GPO5 does come with a lot of the same sounds as the latest release of GIFF that 'current' Finale users get, but...
GPO5 has more instruments. A lot more....
At first it might not seem all that different, and Finale has included quite a pallet of nice sounds that also exist in GPO5, but if you dig around in both banks you'll start to see more variations of each instrument in GPO5. You'll find that there are more key-switches available in a number of the choices. I.E. Where GIFF might only give you one or two solo trumpets to choose from, GPO5 is going to give several solo and player choices, and some of them might even have key-switches for things like mutes and alternate articulations that are not present in the GIFF trumpets.
Another difference is that GIFF only contains 'notation' editions of each instrument. GPO5 comes with a 'standard' bank as well (there ore options at the time you install GPO5 to install either, or both banks). GPO5 also comes with more robust documentation about all of the inner workings of the library, as well as an extra 'standard' version of every instrument which may be more suitable for live performers or tracking style DAWs.
The difference between 'standard' and 'notation' instruments is primarily in where key switches are placed, but also there may be some differences in how pedals and other continuous controllers can work. Notation versions will always place key-switches starting with MIDI note 0, which is WAY to the left of the keyboard (and doesn't even show in the ARIA control panel). Standard instruments will slide those key switches up to just below the lowest octave of an instrument's lower range so they are visible, and easy to reach in live performing situations.
Ultimately, GPO5 is going to give you some extra leverage when building your own sections and ensembles. I.E. If you're not crazy about the stock tutti brass sounds, you could layer up your own custom 'brass sections' in the ARIA player using individual players and overlays from GPO5.