OP: regardless of whether you buy AP in advance (and get a voucher/receipt) or wait to buy until you get there, you still have to go to guest relations at one of the parks or DD to actually get the AP. The reason you might want to buy in advance is that such avoids any possible price increase by the time you go because your voucher is good without paying any later price increase.
As to the above, I do not know how the GAD vouchers work or even if you can use them toward an AP.