It's not an amendment, you just need to file form 3520 and it will be late (if the gift was more than $100,000).
The IRS can impose a penalty but may not, and if they do, you can request a waiver if this is your first time owing a penalty, and for other reasons. If the proposed penalty is substantial, you can retain professional representation. Don't assume there will be a penalty, wait and see if you are sent a bill.
https://www.irs.gov/businesses/small-businesses-self-employed/penalty-relief
