XISF is the native file format that the PixInsight team created. It is an open specification that you can read here:
https://pixinsight.com/doc/docs/XISF-1.0-spec/XISF-1.0-spec.html
In its Image Properties section one may record the gain, which the specification says is measured in e/ADU. Thanks for pointing me to the cameraInfo property, I must have missed that somehow.