No Max Moves have a BP of over 150. 150 seems to be reserved for moves whose base power is normally 150 or higher: self-sacrificial moves, Hyper Beam variants, V-Create, Head Smash, Shell Trap, Eruption and Water Spout.Also, is there a maximum BP a G-Max move can have?
Fighting and Poison moves seem to be capped at 100.
In general, it's like Z-Moves, where there are rough rules but nothing concrete, and each move has its own specific entry for Max Move power.
(By extension, we can assume that Gigantamax moves have the same Base Power as their non-Gigantamax variants.)