They are legally binding — While giving a family member a financial gift may not feel like a big deal to some people, gift letters are not only a formality. They are a legally binding document that both parties must sign.
Your lender may provide you with a gift letter template. If that's the case, you can simply pass it along to the gift giver and have them fill it out. If the lender doesn't provide you with a gift letter template, be sure to verify the gift letter requirements.
Handwritten agreements are somewhat impractical compared to typed versions. However, they are fully legal if written and formatted properly, and are preferable to verbal contracts in practically all cases.
At minimum, a gift letter should include: The giver's name and relationship to the borrower. The dollar amount of the gifted funds. The source of the gifted funds, such as an account number and statements.
Gifts to your spouse. Except as described earlier, you do not have to file a gift tax return to report gifts to your spouse regardless of the amount of these gifts and regardless of whether the gifts are present or future interests.
Gifts to a spouse are not reported on a tax return, regardless of the amount gifted. Generally gifts to a spouse are not subject to the requirement to file a Form 709. What you have described is not an exception so there would be no reporting of the gift on a form 709.