Since the stock was gifted to you, you would have a carryover basis from the original date the person who gifted them to acquired the shares.
If you do not have an acquisition date or basis, you can use "0" for the basis and put "Various" for the acquisition date. (Just make sure that you indicate that this was a long-term capital transaction so that you get the long-term capital gains rate.)
However, you should try to determine a basis amount because this will lower your overall gain on the sale.
You may want to ask the selling broker to search records for clues to help determine basis and when acquired. If the investment was a gift, perhaps a relative got the same gift and has records that show a purchase date.
If those options don’t work, you may have to make an educated guess about when the stock was purchased. You can do that by going to the company’s website or Yahoo Finance to find historical high and low prices for that period. (They should be adjusted for any splits.)