GIMP’s native format is XCF. It is designed to store all information GIMP can contain about an image. XCF is named after the eXperimental Computing Facility where GIMP was authored.
The file format supports layers.
Here is the file type:
$ file sample.xcf sample.xcf: GIMP XCF image data, version 0, 48 x 48, RGB Color
Sample xcf file: download here.