What's the difference between steroids and growth hormone (gnrh) replacement therapy?

A lot. They work in different ways and for different indications.
GH is not GnRH. Gnrh is not growth hormone. Growth hormone is abbreviated gh. Gh is given to people lacking (mostly children) to achieve normal height. Steroids (sex steroids) are given to replace ovarian or testicular function. Some people with pituitary disease lack both types of hormones (gh and steroids) and therefore need both.