How to measure GnRH in menstrual cycle?
GnRH is found in very low concentrations (2-10 pg/mL)

Options:


  
  A highly sensitive analytical method (e.g. radioimmunoassay)
  To sample directly from the hypophyseal-portal system 
  A surrogate measure
  



This abstract: http://www.ncbi.nlm.nih.gov/pubmed/9749566 describes direct measurement.
Direct sampling could only be done under general anaesthesia and in a short window, so I'd avoid that altogether unless you're talking about using an animal model and can surgically impact a cannula.
This abstract: http://www.ncbi.nlm.nih.gov/pubmed/21646369 describes use of the $\alpha$-subunit as a surrogate of GnRH (the $\alpha$-subunit is the same for FSH, LH, and TSH and is released under the control of GnRH).


By the way, GnRH does not promote secretion of hCG so much as it does LH and FSH (LH is fairly homologous to hCG).  For the most part hCG is secreted by trophoblasts (in the placenta) and is quite low except in pregnancy or the presence of certain germ cell tumors, so wouldn't be worth measuring in non-pregnant women or males anyway.

The bigger point is why do you want to measure GnRH pulsatility?  This has been done and the you know that it is pulsatile is because it has been done.