Covers in the 1950's (when the magazine was called Apparel Arts) and 1960's were a varied mix of celebrities (e.g. Rock Hudson, Joe DiMaggio, Robert Goulet) jaunty models dressed like dandies and artistic covers with no models present. Then in the 1970's covers mostly showed couples, with the women more or less in the role of accessory.
Male models in close-up populated most covers beginning in the late 70's and continuing into the 80's. This was considered the magazine's overt gay phase and some say it peaked with the famous "New York Dazzle" cover (far left).
GQ's editorial content was broadened from fashion to cover other facets of a man's life after Conde Nast bought the publication in 1979. This was done to make it more palatable to a wider spectrum of advertisers, especially automakers in Detroit. (And to make it more appealing to heterosexual readers.) When it pitched itself to advertisers GQ more or less instituted a "don't ask, don't tell" policy as it pertained to its gay readership. (I worked in the media department of ad agency Young & Rubicam back then and had a gay boss who loved to make GQ's director of ad sales squirm by asking him pointed questions about its gay readers.)
Over the past 25 years entertainers and sports stars, both male & female,have monopolized covers. (Tom Cruise has been on the cover seven times.) Actresses first appeared "unchaperoned" in the early 1990s (Julia Roberts being the first). By the end of the decade they were appearing regularly (3-4 issues each year) wearing less clothes and showing more cleavage - most likely in response to "laddie" magazines such as Maxim - drumming home the point that "we're not just for gay guys anymore". GQ (whose present editor-in-chief, Jim Nelson, is openly gay) was influential in transforming heterosexual men into metrosexuals.
Finally, some eye candy from the recent past:
To review all of GQ's covers between 1957-2007: http://www.coverbrowser.com/covers/gq