In plain terms
The Gandiva was the bow Arjuna carried his whole life; it made him who he was. Here it slips out of a hand that will not close. When fear runs deep enough, even the thing you are best at can fall away.
There is a quiet reassurance in the verse for anyone who has watched their own competence desert them at the worst moment. It happened to the finest archer in the story, on the biggest morning of his life.