  In grand unified theories with large numbers of fields, renormalization
effects significantly modify the scale at which quantum gravity becomes strong.
This in turn can modify the boundary conditions for coupling constant
unification, if higher dimensional operators induced by gravity are taken into
consideration. We show that the generic size of these effects from gravity can
be larger than the two-loop corrections typically considered in renormalization
group analyses of unification. In some cases, gravitational effects of modest
size can render unification impossible.
