Game Players Nintendo Guide is the general term for the Nintendo-exclusive magazine produced by Signal Research (and later GamePro">GP Publications) under multiple names from 1988 to 1993. It was the first console magazine of the NES generation to hit newsstands (Nintendo Power was subscription-only at the time), and it was Signal's most well-known magazine for most of its existence.
Signal (a company founded by Robert Lock, the former head of COMPUTE! magazine) got its start in 1988 with two separate titles: Game Player's Nintendo Buyer's Guide, and Game Player's Nintendo Strategy Guide.
The exact timeline of these publications is sketchy, as publishing dates generally do not appear in Signal magazines until late 1989. However, judging by advertisements and editorial coverage, it seems that Signal began with a one-off Buyer's Guide sometime in the early summer of 1988, then launched both Buyer's Guide and Strategy Guide as regular publications just a month or two later.
Until early 1989, Buyer's Guide and Strategy Guide were separate magazines, with Buyer's Guide coming out semiannually and Strategy Guide coming out quarterly for a total of six issues per year. (In practice, however, both magazines had a fairly haphazard schedule until mid-1989.) The former offered capsule reviews of hundreds of NES games, while the latter was filled with in-depth strategies. After three (?) Buyer's Guide issues, the two magazines were officially merged in the spring of 1989 and became a single magazine, Game Player's Strategy Guide to Nintendo Games. (One more buyer's guide, the Game Player's Buyer's Guide to Nintendo, was published in October 1989, but this was counted as a regular issue of Strategy Guide.)
In terms of coverage, the magazine's early issues were very close to the Nintendo Power of the era, with heavy emphasis on strategy and very little critical coverage of games or industry news. Although it lacked Nintendo Power's flashy design, its screenshots were larger and it wasn't afraid to cover games right up to the final boss and ending -- which would be derided as spoiling the game today, but at the time was a valuable resource, since GameFAQs didn't exist and Nintendo Power shied away from spoiling final battles.
With the launch of the SNES in 1991, the magazine's name was edited down to Game Players Nintendo Guide (no apostrophe) and adopted fuller reviews, features, and numberical ratings for games. This was the start of the magazine's modernization process, which would ultimately result in one of the most well-remembered editorial teams in US game mags.
Game Players Nintendo Guide and Sega Guide merged after May 1993 to form Game Players, the second incarnation of the publisher's multiplatform game magazine.
Game Players Nintendo Guide Index