Look at websites such as BackIssues or Linda Tresham's House of Magazines to find back issues of "GQ." Dates range on these websites. As of December 2010, BackIssues had GQ magazines dating back to 1980 for $15.95 each. Expect to pay more than the cover price for back issues on sites such as these.
Look on websites such as Craigslist, Amazon and eBay for deals on back issues. You may be able to order magazines from elsewhere in the country with this approach.
Check out garage sale listings in your area. Sometimes people have stockpiles of back issues of magazines they are looking to get rid of before moving, or to clear out some space.
Look in antique stores, second-hand stores or other used goods stores. It's possible the stores have older issues of magazines. Your collection may take some time to develop, but once you have the back issues you want, it will have been worth the effort.