Ok, you ask "Do they even have one?"(digestive track)...Um, they eat , they poop...so thats a digestive track.
The flakes you feed your gold fish don't look like flakes when they come out do they?
Vary your goldfish's diet.  It's healthier and less boring probably for the fish.  Flakes are ok as the staple in their diet, but adding bloodworms, shrimp, peas even bits of oranges and pelleted food will vary their diet some.
Any frozen peas are just fine.  You can take a couple and cut them in half and throw them in the tank.  My fish suck them right out of the shell.  I use a small net and collect the skin or shell later.

still best to shell the peas all the same and easier for them cooked. Zap them in the microwave for a few secs then take shell off. It depends on the size and or age of your fish whether you pop them in whole or mush them a bit. They dont need much. Half to one pea per fish

Oranges are acidid so be careful on that as while the fish like them it can mess up your water (again depends on the size of tank) but bear in mind organges give us Vitamin C otherwise known as ascorbic acid

Try cucumber (slice the rind off for young fish). They love it even more than peas in my experience and it works great for the 'digestive system'

They do have a 'digestive system' but thats not to be confused with the intestinal tract that the food passes through and comes out the other end. Its just that they have a very basic digestive system which isnt like most other fish hence the large amounts of toxic waste they generate

OMG I LOVE ZUCCHINI! Ill have to try that then. That and peas... Peas... (I love this site <3)