How can I making turning more accurate?

I have built a simple LEGO EV3 robot that is 2-wheel drive. I am using the gyro sensor to make movements more accurate. My gyro straight code is working great, but I am having some problems with turning. After many turns (after about 5, there is a noticeable 2-degree difference, which then keeps increasing), it gets off track and ends up far from the endpoint. Here is my current gyro turn code. Do you have any suggestions on how I can make it more accurate? submitted by ...