Please see the link I have provided below. It seems that some sources say that GGT doesnt change, and others say that it increases. The study below says it increases, but keep in mind that the study was performed on rabbits. I have included the excerpt from the article below, as well as a link:
African Journal of Biomedical Research, Vol. 4, No. 3, Sept, 2001, pp. 135-137
INFLUENCE OF PREGNANCY ON SOME ERYTHROCYTE BIOCHEMICAL PROFILES IN THE RABBITS OZEGBE, P.C
The increased intracellular levels of ALT, AST and GGT during pregnancy are contrary to the report findings in the plasma (Karadjole et al 1985; Felbinger, 1987, Allard et al, 1989). These alterations could have been a product of accelerated erythropoietic activity since young blood cells are characterized by increased activities of enzymes (Sass et al, 1964). Alternatively, it could be attributed to the intracellular dehydration induced by increased extra-cellular fluid Na.
http://www.bioline.org.br/request?md01067