After Father activated the nation wide transmutation circle, every soul in Amestris was absorbed into him and used to grasp the moon's gateway. He also opened the Earth's gateway and accessed all of Earth's alchemic power.
Does all of this suggest that all celestial bodies have a gateway, such as Mars or Saturn? And if so, how much power do you think you would obtain from said gateways?
Hmm, I think there's some assumptions in your question that needed to be fixed. First is that Father didn't opened the "Moon" gateway but opened the room for "God" that alchemy presents once there's eclipse, and that's also part of the question.
In ep61, after Father opened "God" gateway in the end of ep60, it's starts with Ed and Al reading in the alchemy book about Sun to be male and present the man specie, while the Moon does it for the female, and their combination (or eclipse) presents complete existence, non-male and non-female. Ed concludes it's not something just incredible but "God" (since God can't have male/female references).*
Backing to ep26, Ed explains the transmutation circle in Gluttony's tummy "Drop God to the ground and become perfect existence". al this coming together to the final events of FMA09 where Father fulfil that purpose, but to gain such power to pull out "God" he needed to open Earth gateway to achieve great power, only to pull out "god".
From here I will make some speculation, but based on above references they'r probably be true.
First of all, I fixed what you claimed to be "Moon gateway", we see that alchemy doesn't treat it that way, and more than that, though Father explained in ep60 that Earth has it's own gateway it seems that it used as you said because the power of alchemy. Once we will take the explanation of the alchemy source, it won't be far to say that even in other planets like Mars there's a gate, but the problem is the price - in order to open Earth gate Father needed to sacrifice the whole Amestris, who you gonna sacrifice opening Mars gate? it's empty of people.
Either than that, because Alcehmy FMA09 presented, it treated only the eclipse as "perfect existence" you wouldn't able to gain "God" powers from Mars like from the eclipse but only as much info the planet have, and unlike earth that contains people using alchemy, Mars gateway may not be strong as earth.
So though there's no evidence for other planets gateway it possible, FMA09 don't treating those beings because there's probably not much point in those, or the other way around - there's no gateway.
Once again, all this speculations based on the above, hope I answered much of your question.
*All the translation from the Anime is from non-english source, if there's some other version I will be glad to be enlighten.
