Guppies give live birth where goldfish lay eggs, so I doubt they are compatible. Most likely your goldfish is eating a lot of food and growing fat.
Here are some ways you may be able to tell if you have a female or a male goldfish:
The female will generally have a fuller abdomen. If you view the fish from above the aquarium, the female will be fatter on one side of her body than the other. (A male fish tends to have a more even shape when viewed from above).
If you look closely, the male goldfish will develop tiny tubicules (Little white dots that look like tiny pimples) on the gill covers, and along the front edge of the front fins.
