Kindly clarify the following:-What to say according to Sunnah before and after good gathering? Please give evidence.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Meeting with people is not recommended except for what is good.
Allah Says (interpretation of meaning): {There is no good in most of their secret talks save (in) him who orders Sadaqah (charity in Allâh's Cause), or Ma'rûf (Islâmic Monotheism and all the good and righteous deeds which Allâh has ordained), or conciliation between mankind, …} [4:114].
So, a Muslim is advised to meet Muslims in places of performing obedience to Allah such as Friday prayer, congregational prayers, knowledge sessions, inviting and guiding others, and similar good deeds.
Furthermore, our meetings should be for what is beneficial; Zikr of Allah should the focus of such gatherings. Abu Dawood and others reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Those who attend and then leave a company in which there has been no mention of Allah, they leave it like dead (i.e. without benefit) and it will be a source of sorrow for them" .
According the Sunnah of the Prophet Muhammad (Sallallahu Alaihi wa Sallam), one is advised to end a meeting with the following sound Du'a: Abu Dawood reported from Abu Barzatah al-Aslami who said that when Allah's Messenger was about to leave company, used to supplicate: "O Allah! Holy You Are, and all praise is for You: I bear witness that there is none worthy of worship except Yourself: I ask for Your forgiveness and turn to You in repentance" .
Allah knows best.