Like any surgical procedure, a gastrojejunostomy could result in infection or another complication. A gastrojejunostomy connects the stomach to the jejunum, bypassing the duodenum, the beginning of the intestine at the bottom of this image.
One more thing: Don't make your turtle's water temperature too warm. For most adult aquatic turtle species, 78 F. (25.5 C.) is the highest you want unless you're treating them for an illness and a veterinarian has told you to raise the temperature. Otherwise, too-warm water can encourage bacteria growth in the tank. It also discourages turtles from basking sometimes.
The rule of thumb is 10 gallons of water for 1 inch of shell. Since your turtle is 3 long, he needs 30 gallons of water. For him/her to grow, it needs a larger tank. Reptiles will only grow as large as their environment will allow.