You use gpu_push_state() and gpu_pop_state() when you want to use any of the gpu_set_*() functions to draw a certain way, but want to quickly revert to the setting before it later without explicitly knowing what it was before. It's a faster alternative to using gpu_get_*() beforehand.
General form:
Code:
// Remember the original way
gpu_push_state();
/* Call gpu_set_*() functions here */
/* Special drawing here */
// Revert to the original way and continue
gpu_pop_state();