I *partially* agree with MovieMe's answer. It was an autolock gate. However, Neil **DID** unlock the gate (or pick the lock if you will), just not when we all think he had to. In the forward timeline, Ives (after regaining conciousness and passing the gate) saw Neil closing the gate behind them and reverse-running away, which means that in Neil's perspective (reversed timeline), he HAD to pick that lock when he arrived at the gate (cause it auto locks, remember?) just to get in and hold it open waiting to close it again before finally getting shot.