Nobody on here is Native American. Native Americans have better things to do rather than see what's up here.
American and Mexican are nationalities. Mexico has (among others) Mestizos, Hispanics, and those who are more closely related to the indigenous peoples of North, Central & South America (everyone who truly has Native American ancestry).
Gaethje considers himself half Mexican and half German (not very scientific if he's talking about genetics, btw, barring incest that's like someone thinking skin color has more to do with the entirety of genetic make up than it really does in someone who is mixed) but I'm pretty sure he's not talking about genetics, and is referring to heritage.
He has mixed heritage that includes heavy Mexican-American influence by way of his mother and Arizona. As well as the other side of Arizona's influence on his upbringing, like his father and those that are related to him through his father, and any number of his friends who don't happen to be Hispanic/mestizos/etc. or in touch with one, or more, or any heritage they could have been.
He's just like everyone else around the world who doesn't actively fight the influence of "exotic" or "foreign" heritage. He's been eager to wear the United States Flag too, if that removes any inexplicable sting a basement dweller might weirdly decide to feel about this discussion that is obviously suppose to be innocuous.
His genetic make up? lol, ask him to do 23 & Me or some shit, and to post his results.
To give you an idea, I'm pretty sure his mother lovingly refers to him as "mijito" which translates to my boy/my son/my little one or w/e. & If Justin & his dad are still actively connecting to their German roots, there might be a chance Justin refers to his dad as "vater" which does not mean waiter