"GLD-1 is an RNA binding protein which binds to the 3' UTR of the MES-3 mRNA. If GLD-1 is not functional, MES-3 mRNA levels do not change, but MES-3 protein levels increase. The size of the MES-3 protein does not change, only its quantity. What is the function of GLD-1

GLD-1 is a regulatory protein whose function is to regulate the expression of MES-3.

Explanation:

Since the MES-3 protein itself is not modified but only its quantity is, then it is likely that the GLD-1 protein is only inhibiting MES-3's expression; keeping it at a certain  level set by the organisms DNA.