Just now you can only generate a thank you letter from within a gift record. We are increasingly thanking donors by email and just now you have to go into Actions, create the thank you email and then back into the gift to mark it as acknowledged. It would be ideal if the email could be generated from within the gift record and it marked it acknowledged the same way it does with letters.
Note: It is possible to create the letter then email it and when you send the letter and return to the gift it marks the gift as acknowledged, but it doesn't pull the email address through. It is a process that involves quite a number of steps so isn't user friendly.