I want to add a type of transaction to my investment accounts for charitable donations

I think there are a few approaches to this. Hopefully someone with more knowledge in this area will jump in to correct or add to my thoughts...

Was the gift from a taxable or non-taxable account?

If it was from a non-taxable account like an IRA, then it seems you could use two transactions: (1) Sell the shares, then (2) use a Payment/Deposit transaction to record the payment to the recipient, using a category such as Gifts&Donations:Charity. Although a sale of stocks would normally be a taxable event, doing it inside a non-taxable account in Quicken will prevent the capital gain from showing as taxable income.

In a taxable account, you could instead use Remove Shares, followed by the Payment/Deposit transaction. Remove Shares will prevent any capital gains on the shares from appearing.