I forgot to file a gift tax return with my 2018 tax return. In 2018 I contributed >$15000 to my child's 529 plan (but less than $75000). What should I do?

[DoninGA]

Gift tax returns, Form 709, are not included with your personal tax return Form 1040.  

The Form 709 is due on April 15, 2019 for tax year 2018

IRS Form 709, United States Gift (and Generation-Skipping Transfer) Tax Return - https://www.irs.gov/pub/irs-pdf/f709.pdf

IRS Form 709 Instructions - https://www.irs.gov/pub/irs-pdf/i709.pdf