Gateway timeout Error while Load Testing: User Login-->Select Team--> Performs direct chat-->Logout

Hello
We have performed load testing using JMeter for 100 users. For the first few requests everything works fine but after sometime Gateway timeout error is encountered.Please refer to screenshot: http://imageshack.com/i/pnnrBje4p

In the beginning, same APIs run successfully but later the very same APIs return this error (504 Gateway Timeout Error).

We have configured mattermost with AWS server .
AWS server details:
Web Server Instance
Instance Size: C3 2X large
RDS Server Instance
db.m4.large