Naked Quad

The only potential values for the four cells G4, I4, I5 and I6 in box 6 are the four values 1, 5, 6 and 8. Because the number of values matches the number of cells, and each cell must have a value, these four values are "locked into" these four cells, in any order.

Hence other positions of these four values can be removed from box 6.

Therefore we can removeG6-1.