While the departures from GMP are doable, there are a lot of things that have to go right.
Flight has to arrive on time or early.
Bags have to come out quick, if checked in.
No delay at immigration.
Short wait for bus to GMP. Or take a taxi, but $$$.
Smooth check in and short security line at GMP.
In the end, is it worth all that stress to get to Tokyo a few hours earlier?