The metabolic profile reported here is unique to GTPCH deficiency or a defect in the GTP activator protein. While there is no known demonstrated defect in this protein, it would cause similar functional deficiency. Other enzymatic defects in the pterin synthetic pathway present with very different metabolic profiles.
I googled but could not find a particular "activator protein" that interacts with GTP cyclohydrolase I. Is this one of the GTPase-activating proteins? They must be numerous, and I haven't found one that was shown to activate GTP cyclohydrolase I.
At the same time, the authors use the definite article, as if this protein is known. I'm intrigued.
I found this paper, in which GTP cyclohydrolase I is said to interact with Rho-GTPase-activating protein 7 (also known as DLC1), but the paper does not say that this protein is an "activator", they merely list it as part of the putative transcriptome of GTP cyclohydrolase I.
Further, in the quote the authors say that "there is no known defect in this protein", but the OMIM page for DLC1 says that a defect was known from at least 2000, although it is related to cancer, not to the BH4 pathway.