Why are the Vishnu Silas found from Gandaki river called 'Salagrama' Silas?
I have found one reference in the Varaha Purana.
Shalagram got his name from a Muni called Shalakayan. Or it got it's name from a type of tree called Shal tree (Shorea Robusta).
Quoting from this PDF:
Prithvi requested lord Varaha to shed light on the famous penance which sage Shalankayan had performed. Lord Varaha narrated the following tale- "Long long ago sage Shalankayan did an austere penance to please me (lord Vishnu). There was a 'sal' tree nearby and the sage was ignorant of the fact that I dwell in that sacred tree. When I appeared, he requested me to bless him with a divine son who had qualities like lord Shiva. I revealed to him that his son (Nandikeshwar) had already manifested and was presently staying at 'Vraja' (Mathura). I also revealed to him that although I was present in the 'sal' tree but he was unware of this fact because of his ignorance. In course of time, this very place became famous as 'Shalgram tirth'. A devotee who makes a pilgrimage to this holy place is liberated from all kinds of worldly bondages.
To add to Rickross, the Ahnic-Kritya quotes from Vishnu-Dharmottara-Purana
Salankayan Muni while meditating on Vishnu saw a Shala Tree appearing in front of him. Sri Vishu appeared under the tree and told him : I am going to appear in the Gandaki river as shila.So the Silas are known as Salagrama.