Sura (सुर) means, a devtaa.
Sari (सरि or सरी), is to be taken for the word Saritã (सरिता) i.e, a river.
Since Gangā is said to be flowing in the swargaloka first, and thus it is always in vicinity of gods (सुर), hence, it's also called as Sursari.
Again, as per several stories in the different puranas, it is in some way or other, connected to all the three deities of the Trimurti Godhead. They are also to be called great sura (सुर), - Brahmaa. Vishnu, & Shiva.
And thus, owing to her connection with several different gods (suras - सुर), Gangā, is thus called surasari (सुरसरि)
- Some explicit references to Gangā as Sursari.
As Goswami Tulasīdāsa says in a couplet in Bala Kanda
कीरति, मणिति, मेलि सोई।
सुरसरि सम सब कर हितु होई।।
Meaning - Popularity, Prose and Pelf, these three are only worthy if and only if, when they are as auspicious and useful as the River Ganga (Sursari)
Further, in the Rāmacharitamanasa, Ayodhya Kaanda, [1.2.132] , he makes references to Ganga as Sursari, which is river Ganga only, called by Mandakini (मंदाकिनी - meaning, one that flows slowly (मंद) ), that flows in the Heaven (swarga) realms.
नदी पुनीत पुरान बखानी। अत्रिप्रिया निज तप बल आनी॥
सुरसरि धार नाउँ मंदाकिनि। जो सब पातक पोतक डाकिनि॥3॥