what should be the minimum time gap between azaan and namaaz in congregation for the maghrib prayer

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

There is no minimum time gap between the Athaan and Iqaamah of Maghrib prayer. It was authentically narrated that "the Prophet, sallallaahu ‘alayhi wa sallam, used to perform Maghrib prayer after the sun had set when its upper side would disappear…" [Ahmad and Abu Daawood] Another narration reads: "When the sun sets and disappears into the curtain of darkness…" [Muslim]

However, it would be preferable that the Imaam takes into consideration the circumstances of the congregation and delays the Iqaamah for the period of performing two Rak'aat before Maghrib prayer. It was authentically narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Perform two Rak‘ahs before Maghrib; Perform two Rak'ahs before Maghrib." In the third time he added: "for whoever wills," fearing that people would consider it a Sunnah. [Ahmad]

It should be noted that the time for Maghrib prayer extends until the absence of the twilight because the Prophet, sallallaahu ‘alayhi wa sallam, said: "The time of Maghrib prayer continues as long as the twilight has not disappeared." [Muslim]

Allaah Knows best.