Revell had a chance of his own after 20 minutes, but the experienced forward could only clip the outside of the post with a powerful header.
Rotherham had the lead 15 minutes before the interval, however, when Agard headed a Joe Skarz cross into the back of the net from close range.
The Millers continued to dominate in the latter stages of the half and came close once more through Agard, but his header was straight at Gaston.
Chances were few and far between in the second period, but Febian Brandy placed a close-range effort wide for Rotherham just past the hour mark.
Richard Wood was next to come close for Rotherham after 72 minutes, but his header was straight at Gaston.
Greenock, however, largely struggled to create clear openings in front of just under 600 spectators in South Yorkshire.