So, there was a Gundam RPG that used a modified version of Mekton Zeta ruleset. However, it was only released in Japan. It seems to cover the One Year War exclusively. I'd like to dump some translations of the rule changes to Mekton Zeta. It's split into multiple PDFs for different types of rules and different mobile suit types, so it might take a while. Still, if you have Mekton Zeta, you should be able to play Gundam Senki by using these.
Full credit to the translation goes to user Mendou on Gundam Forums. Tomino bless them.
>>50007404
I should mention that apparently the the rules for ships are missing. They haven't been translated yet and it seems like Mendou might not wanna do them.
>>50007629
Zeon character template
>>50007662
>>50007662
Feddies
>>50007675
>>50007730
GMs and their variants
>>50007745
And here come the zakus
>>50007760
GANDAMU
>>50007798
>>50007854
>>50007862
>>50007974
>>50008003
>>50008014
Actually, screw that. Here's all of them
https://www.mediafire.com/#lq3j7aj5hpuxm