GULO only for mammals?
GULO encodes L-gulonolactone oxidase which catalyses a step in the biosynthetic pathway to ascorbic acid (vitamin C).

I did a BLAST search at NCBI using the mouse GULO protein as probe, excluding mammals, and I saw highly significant hits in reptiles, birds, sea urchins, fungi and bacteria. I conclude that this enzyme, (or very close relatives) is ubiquitous.

Humans (and other primates; also guinea pigs and the Dalmatian coach hound) have a defective GULO gene and so require dietary ascorbate. 

Since the gene is non-functional in humans it isn't expressed in the foetus.