To do this you are going to need to use the ACF JS API to update the field value. There is a lot of things that ACF does besides updating the value.
I do not know the details of updating the value of a gallery field using the API. I would start by creating a test script that gets the value of an existing gallery field with images in it and logging that value to console and then mimic what I get in the test when attempting to update the value.
