GER works by erasing the effect but keeping the cause. It resets things to 0, making it as if the effect of your actions never happened. The death loop works in that you go through the experience of dying (the cause), but you never die (the effect, being dead, is erased), and since you don't die, you experience dying again (reset to 0). This resetting concept can be applied to virtually everything.
GER also works independently of Giorno, so matters of Giorno's reaction speed or how quickly his powers is countered is moot. Only way to beat him is reality warping hax (like with The World Over Heaven) or causality-breaking hax, to be free from GER's resets and loops.