I should have filed a Form 709 with my 2018 return for a gift over $15K. Should this be filed as an amended return or filed some other way since it is only information?

Form 709 is not an income tax form and would not have been included with your tax return.  TurboTax does not support or provide Form 709.

Money that you receive as a gift is not taxable income to you, and you do not need to report it on your income tax return.  Money that you gave as a gift to someone else is not deductible for your taxes.

Turbo Tax does not support the gift tax form 709, but here is a link:

https://www.irs.gov/pub/irs-pdf/f709.pdf

https://turbotax.intuit.com/tax-tips/estates/the-gift-tax-made-simple/L5tGWVC8N

https://www.forbes.com/sites/ashleaebeling/2017/10/19/irs-announces-2018-estate-and-gift-tax-limits-11-2-million-per-couple/#3226e75d4a4b