I am not a vet... but GnRH is supposed to stimulate the pituitary to produce more of the 2 hormones (FSH and LH, I think) that strengthen the egg and cause it to be released from the ovary. It supposedly speeds up the release so that an egg should be released within several hours .... I am not sure of the effects on the actual heat cycle or the strength of the heat... I think that is more something like lutelayse or cysterolin or something like that, will do by disrupting the progesterone and causing the CL to "dissolve" off the ovary so it thinks it is not pregnant and prepares to produce another ripe egg/follicle.
I have next to no experience with using cdir's and such...
If the vet said to give her a shot of GnRH then it would be my understanding that there was/were an egg/eggs on the ovary that had not been released... and it might have been to just get her to release the egg which would get a CL to form and start through the 21 day cycle and then she should dissolve the CL and go into heat again since she is not pregnant. I had heard an old vet one time say to give a shot of GnRH to a cow they didn't see in heat,,, then should come into a regular heat because her ovary had gone from "static to active"... sorta like jump starting the system that had gone into "hibernation" .... but the function of GnRH is to cause the pituitary to produce the hormone to get her to release an already mature egg on the ovary...
Please Ken chime in because I might be saying it wrong.