I don't see a way to have different thank you prints for different galleries. Did I miss something? If not, would this be possible?
There's currently no option to set up gallery specific thank you prints. You may only set up a thank you print for your entire site nad toggle that on/off.
Could you add in more detail on how you would use such a feature and why it would be important for you to have? With some more details on why this would be important to you, we could file a feature request for you.