What does GRAT stand for?

Gratis - Free - Latin (ROG)

Gratuity (AABC)

Gray-Backed Tern - North American bird banding code (BIBA)

Gray Thrasher - Common English name - Scientific name: Toxostoma Cinereum - American Ornithologists Union four-character bird identification (IBP)

Grattan's Virginia Reports - A publication (DLA)

Graticule (SAUS)