Shaun Batt netted a hat-trick to continue the London club's remarkable start to the season which sees them top League One with five wins out of five.
They took the lead when Batt scored from the spot and he grabbed his second a minute before the break.
He slotted home a third before the hour and although Bradley Dack headed a consolation for Gillingham, Orient's impressive start to the campaign continued.
"With the momentum we've got and the spirit we've got and the football we are playing at the moment we are looking sharp," said Slade.
"It's not just the finishing - we're pretty ruthless in both boxes.
"It's not just about scoring goals, it's about doing it right in both boxes and we've done that really well."