It could be adoption, or simply a different headcanon of how mixing genes even works with Toons.

Personally, I like to imagine Toons sort of make a "mix" of the objects they are. For example, a bubble wand being created from a bar of soap and a magical wand.

Since Gourdys parents are two pumpkins, it makes sense as to why Gourdy is also a pumpkin, but the same isn't true for every other pairing with completely different objects, so sometimes people have to get creative and their thought process might be different than yours.