Question about Cost Basis on Stock Power Form

My sibling and I were gifted stocks when we were young, with our dad as a custodian on each of our accounts. We are both finally submitting forms to have him removed as custodian since we are both in our 30s, and it's long past due. The portion of the Stock Power Form that we are both unsure about is the cost basis portion. My options are Private Sale, Gift, and Inheritance. Would I be assuming correctly that since the shares were gifted to us over 20 years ago that we would select Gift on the forms? I tried calling the IRS to find out more information only to spend 10 minutes navigating a phone tree, and to be ultimately told they couldn't answer my question. I didn't even get to talk to anyone. Neither of us are planning on selling any of our shares at this time, we're only transferring ownership solely into our names without our dad as a custodian on the account. We have both of our accounts set up to reinvest any dividends, but haven't made any sales on either account. Any guidance would be greatly appreciated!