Chinese kinship is exact:
姑姥姥 = the sister of your maternal grandfather (
姥爷). Compare
姨姥姥 (yílǎolao, maternal grandmother's sister),
姑奶奶 (gūnǎinai, paternal grandfather's sister), and
姨奶奶 (yínǎinai, paternal grandmother's sister). English just calls them all 'great-aunt.'