After you file

From your question:
"I was gifted $100,000 in 2017 from my father. We were told due to his lack of income . . . "

While there was an obligation to file the Form 709 Gift Return prior to the filing of your father's Form 1040 for 2017, it sounds as if your father, whether individually or jointly, has possession of an estate that is less than $5.6 million (2018 exemption) in total value, at this time.

N.B.: 2017 amounts were
a) Lifetime Exemption $5.45 million and 
b) annual single person exclusion @ $14,000.

As was stated in TurboTaxMichaelL1's answer, but to clarify this point, the usual Form 709 filing simply reports that of the entire Lifetime Exemption (the $5.6 million in 2018 - it was minimally smaller in 2017) the Gift amount is to be accounted for against that.  However, this is further reduced by the fact that of the $100,000 Gift, your father could and can legitimately characterize not less than $15,000 as an excluded gift.  Gift exclusion in 2018:  One person can give another person (not a referring to a spouse) as much as $15,000 in one year with the Gift being excluded from the obligation to report on Form 709.  If a pair of parents Gifts, then it would be twice 2*$15,000; or if a parent gifts to a married child (the child and spouse), it would be twice.  Thus, it is possible that not less than $15,000 would be excluded and if both parents were surviving and the recipient was married, that would exclude $60,000.

Lastly, as Michael said, no Gift Tax is due, if all gifts made during the Lifetime, total less than the Lifetime Exemption, so no penalty is assessable and there is no practical need for a Form 709.  However, if the gift is to be repeated, good practice suggests that retroactively for the prior gift, and going forward for future non-excludable gifts, Forms 709 be submitted.