GT refers to a coated secondary fibre board.
Front top layer = hf white or light hh white
Inlay = grey
Back cover layer = light
hf white: wood-free white layer of bleached pulp with a maximum of 5% mechanical pulp
light = light layer of mechanical wood pulp or of a mixture of unbleached pulp and mechanical wood pulp
grey = layer of recycled pulp (reused cellulose and groundwood fibres)