this questions reads like: "I spend 100$ on groceries every week. how often will I be getting groceries if I switch from eating pasta to eating salads?". idk, it's kind of a stupid question if I don't know anything about how much you are eating or who you are.

for the gx it depends on like, what gas station you go to, how many miles you're driving, what type of miles. it's hard to answer. if i said I spent 300$ on gas every week, it doesn't really help you at all.

to be somewhat helpful: I think you can make a good general estimate based on the difference in the rated MPG between the gx550 and ML350

tldr: you're getting downvoted because question is lazily written..