Mongia struck two sixes and eight fours on his way to the half-century as his side, virtually assured of victory now, declared at 354 for two. The Punjab left-hander had hit 111 to take their first innings total to 375.
In reply, Gloucestershire made 234 and 48 for one on the third day of the match. In their first innings, Kiwi paceman James Franklin took four wickets for eight runs to return figures of 5-58.
The 27-year-old Indian, who has come in place of Australian all-rounder Stuart Law, is playing his second stint for the club this season. Earlier in June, he had temporarily replaced Carl Hooper after the West Indian had broken his thumb.
Indian pacer Ajit Agarkar, playing for County side Middlesex, also proved his worth as he plucked three wickets giving 72 runs to restrict Worcestershire for 305 in County championship Division One match at New Road.
The Mumbai player sent to pavilion Andy Bichel, A Hall and M S Mason. In reply, Middlesex were 260 for three in their first innings.