There is technically no actual dollar penalty for filing a gift tax return late unless gift tax is due (although leave it to the IRS to try to assess something).
However, filing a return starts the running of the 3-year time period for the IRS to challenge the valuation(s) reported on the return (if you never file the statute never begins to run). Also, although probably irrelevant here, you lose the ability to allocate the GST exemption on a late file gift tax return based on its value on the date of the transfer.
