Quoted from edednedy:

Contrary to popular belief. All manufacturer schematics released before 1976 without a copyright notice are in the public domain, including Gottlieb. They were public domain the day they were released without a printed copyright notice and date. Gottlieb began adding the notice around 1973 or so. I have many pre-73 Gottlieb schematics without such notice.
What happened was Gottlieb Development LLC asked sites such as IPDB to remove the copyrighted material such as manuals, schematics and roms. They were removed as requested. But they threw the baby out with the bath water and removed everything. Including the public domain stuff that was never copyrighted to begin with.
Gottlieb Development LLC is a litigious bunch who once sued Paramount Pictures for copyright infringement because a Gottlieb machine was used as a set decoration in the background of a movie. They lost. But the pinball sites don't have Paramount's lawyers. So no one wants to risk them sighting intellectual property or some nonsense, and get sued, just to make a few schematics available to a niche section of the pinball community.
... And that is why you can't download public domain Gottlieb schematics.