I went to GameStop today to finish paying off my GTA V pre-order. (I got the Special Edition.) While I was there, I asked about the details on the midnight release of GTA V.
The store associate told me that at 8:00pm (your local time), GameStop will close. At 10:00pm (your local time), GameStop will re-open. When they open, GameStop will begin taking receipts or giving out receipts, if you do not have a receipt; with these receipts, employees will write a number on your receipt or have you take a number, and both of these methods will allow you to hold your place in line.
After obtaining your number, you may leave the store and come back at around 11:00pm (your local time) to wait for your GameStop to give out copies of GTA V. While waiting, GameStop will be giving out prizes and whatnot.
As a suggestion, I advise anyone who is going to the midnight release of GTA V to arrive at their local GameStop at 9:00pm (your local time) to secure the best place in line to obtain one of the first few spots in line.
P.S. My GameStop said they have 275 pre-orders, in a town of about 30,000 people, so yours may have a larger or smaller number of pre-orders.