Since the amazon.com description doesn't give too many details, here's what you get with the d20 Call of Cthulhu Gamemaster's Pack: A tri-fold GM screen, four handy bookmarks, a booklet of official-looking forms, and a largeish booklet that comprises the Gamemaster Pack proper.
The screen is complete and stands up well, but is slightly smaller than most screens I've worked with. The side facing the players features the incredibly cool artwork for Chaosiums current edition of the Call of Cthulhu RPG. Unfortunatley, the screen doesn't have the game's skill list, so you'll need another method to keep track of which skills can be used untrained and other skill details. Other than that oversight, the screen is so handy that I hardly have to consult the rules book while playing.
The bookmarks are nice and functional. Each is customized for a different section of the rules and features a mini-index of relevent information. Between these and the information on the screen, finding information during a game session is not a problem.
The book of forms is rather useless to me. While it is nice to have official looking forms (dental records, asylum records, etc.) as play aids during a game, these are all customized for an Arkham setting - little use in my Chicago-based games!
The main Gamemaster Pack book is largely comprised of a nice adventure, "The Lost Temple of Yig". The 30 page strikes a good balance between adventure and exploration. Game stats are given for both the d20 and the original version of the RPG. Further background for "The Lost Temple of Yig" or your campaign is given about its Vanguard Club. The official errata follows, which isn't as bad as you'd think - it just seems that some of the authors of the original book didn't communicate too well as most of the errors are internal inconsistancies. A conversion system for using original Call of Cthulhu material with the new d20 version is pretty straightforward - you determine the level of the character you're converting and buy skills and feats from there. Next up are the record sheets, which along with the GM screen, are the shining stars of the pack. There is a redesigned two-page character sheet (with an old-school insanity track), minor character sheets, and monster master and minion sheets. The graphic design of these sheets (and the whole package) is outstanding.
Overall, while I would've preferred paying a little less for just the screen, bookmarks, and sheets alone (errata should always be free!), the adventure and background information is good enough that [this]is a bargain for the whole package. More than a screen, this is a pack to make any d20 Call of Cthulhu Gamemaster's job easier.