Lasimant, who only came off the bench in the 86th minute, headed home from close range to sink the Gills, who had taken a fourth-minute lead through Adebayo Akinfenwa.
Cody McDonald had hit a post in the opening 90 seconds for the hosts, while Larkins - on his league debut - made good saves to prevent the Gills striker and Jake Hessenthaler from extending the Kent side's lead.
After the break Nelson then made a brilliant save to deny Dean Cox from 20 yards and Legge cleared Lasimant's 85th-minute volley off the goal-line.
Orient had temporarily been knocked off top spot with Brentford's early victory over Swindon but Lasimant's second goal of the season ensured they were not away for long.
- Sports & Recreation
- Adebayo Akinfenwa