When Google first launched Play Magazines, there was an immediate backlash from users who were already subscribed to paper magazines that offer a free digital copy, as there was no way to grab said digital subscription from Google. Instead, these users were treated as if they had no prior subscription at all and were effectively forced to pay again if they wanted to read on their mobile device. Needless to say, that didn't go over very well.
Fast-forward to now, and you can finally get your digital fix from your analog subscription straight through Google Play. For magazines that offer such a thing – like Wired and GQ, for example – there's a new entry under the "subscribe" option: "Free for print subscribers." After selecting this entry, it's pretty cut and dry – just enter your current subscription info and you're good to go.