I found this amazing blog with step by step instructions on how to put a gastronomy tube into a doll. I spoke with the author of this tutorial and she gave me some great websites where you can purchase dolls/stuffed animals with a gastronomy tube already in place incase you dont have the time to make your own. 

• Mini Buddy https://www.facebook.com/myminibuddy
• Tubie Friends http://www.tubiefriends.org/
• Tube Swap https://www.facebook.com/TubeSwap

Source: http://agirlandhertube.blogspot.com/2012/01/how-to-put-g-tube-into-doll.html