I *partially* agree with MovieMe's answer. It was an autolocking 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 (because it autolocked, remember?) just to get in and hold the gate open for his mates to get out.