How you catch it is you have to fall behind it in order to battle it. I recommend to save before falling back in so as to be able to catch it by surprise. You have to leave the pokesnax there and then come out and back in again to where gible is and just save(before the sinkhole) before going back..so if he is not there you can just soft reset.( I cant explain it I'm not good with explanations) Hope this helps