You can use Radial Fountain Fill or Blend tool. Check this tools out with built in tutorial or built in help within the program. In my opinion with gradient design you can go either way. Check out what is the most economical. You can gang a custom transfer or screen print it. Gradient can get expensive depending on how many steps there are in the gradient. I do not do screen print but I think technically each step means one corresponding screen frame. Depending on how many shades of different color are in the gradient. I could be wrong. Or you can go with digital transfer if the shirt is light colored.