GnRH production is increased by Clomid and Novaldex. So "using" GnRH is not much different from using Novaldex.
HCG is a completely different hormone. HCG (Human Chorionic Gonadotropin) has actions similar to LH (luteinizing hormone), but it is far more effective at treating hypogonadism.
Problem is: it may also increase production of Estrogen.
So, to summarize, for PCT purposes:
Novaldex / Clomid, besides being anti-E, increase GnRH which increases LH which increases T production (to an extent)
HCG directly stimulates the testes, making them grow and produce T, however you might get gyno
You might combine the two and get the best of both worlds (anti-E + HCG) but that may make you grow pink furry bunnies out of your ass (meaning: I have no idea if that has been done and if so what are the side effects).