Gustavo and Germain netted in the space of two minutes to put OM up to fourth on 51 points from 32 games, five points behind Olympique Lyonnais who occupy the last qualifying spot for Europe's elite competition.
Nimes, who reduced the arrears through a Teji Savanier penalty eight minutes from time, are 10th on 43 points.
St Etienne will leapfrog OM into fourth place if they avoid defeat at home against Girondins de Bordeaux on Sunday, while leaders Paris St Germain will claim an eighth title if they grab at least a point at second-placed Lille.
Marseille lost striker Mario Balotelli to a thigh injury as the Italian was replaced by Germain in the 32nd minute.
It was Germain who opened the scoring in the 72nd minute with a header from Dimitri Payet's corner.
One minute later, Gustavo doubled the tally with a shot from close range after slaloming through the Nimes defence.
The visitors reduced the arrears when Savanier beat Steve Mandanda from the penalty spot after a handball from Boubacar Kamara.
They thought they had earned a last-second penalty when referee Frank Schneider judged there had been another handball, but a VAR review overturned the decision and OM escaped with all three points.
(Reporting by Julien Pretot; Editing by Clare Fallon)
