If you're a casual fan of old Aerosmith and only own "Greatest Hits", then this disc is a perfect follow-up. "Gems" spans the same classic years as "Greatest Hits", without repeating any of the same tracks.
You get two more tracks from "Get Your Wings" (Lord of the Thighs, Train Kept a-Rollin'), three more tracks from "Rocks" (Rats in the Cellar, Lick and a Promise, Nobody's Fault -- this last one having just been covered at the time by Testament), two more from "Toys in the Attic" (Adam's Apple, Round and Round), and one more song each from the rest of their studio catalog with Columbia Records (No Surprise, Mama Kin, Critical Mass, and Jailbait). They certainly went for more of their heavier songs for this compilation.
But the main reason people were buying Gems at the time was for the remaining track, "Chip Away at the Stone". Until then, it had only been released as a single. The studio version had not been released on any album. Some time before the release I remember Mtv's Headbangers' Ball playing the video for it, which revived it as a hit and raised the anticipation for "Gems".
So if your copy of "Greatest Hits" isn't enough, but "Pandora's Box" is too much, then go for "Gems".