My mother wants to give me a gift in excess of $100'000. She transferred money from his non us account to his us account and wants to transfer that money to my us account.
She is a non us citizen, will I be subject to gift tax? If the money was not earned in the US but is now in the US?

If the giver is NOT a U.S. Resident, no gift tax will be due.

However, a transfer of that size will have to be reported on Form 3520, though no tax will be due.

While ATE is correct. I'm confused by the use of pronouns here. Hopefully this is just a language issue. But you say your mother wants to give you a gift, then you say "She" transferred from "his" non us to "his" us accounts. That should be her not his.