그런 경우 vs 그럴 경우
The short answer is that they both mean "(in) such a case" or "(in) cases like that" and can be used interchangeably.
There is a nuance difference which makes 그런 경우 sound like it's about a concrete, existing case and 그럴 경우 a hypothetical one. But this difference is small and too subtle, so I think it's okay to consider them to be the same.
The difference between 그런 and 그럴 involves several key principles of verb and adjective conjugations and their conversion to attributive adjectival forms. Unless you have some specific question about it, it's a quite big subject to talk about.
Maybe you can leave the question open (don't select the best answer) for a while, so someone else or I myself can add more information later.