Both of the other answers are correct and good answers, but I think neither directly answers your question.
No, you do not need to pay additional taxes on the wedding gifts simply because of the fact that they are going into a Roth IRA. Similarly, if you put them into a traditional IRA, that amount would be deductible (assuming you met the other criteria, including minimums and maximums of earned income, in both cases). The act of putting money into a Roth IRA is not what makes it taxable; its original source is. Roth simply does not reduce your current taxes any, whereas a traditional IRA would.
The seeming exception to this is when rolling money from a tax-deductible source to a non-tax-deductible destination, such as transferring money from a Traditional IRA or 401(k) to a Roth IRA or 401(k). Then, the taxable event is really the distribution from the Traditional IRA or 401(k), not the deposit into a Roth IRA or 401(k), though of course if you rolled a 401(k) over to a traditional IRA it would not be taxable.