refer to the pictures, there is a big gap between the roof and gutter, rat going in/out to my ceilings through the gap, I've tried to lay some net but it is hard to fit it, there is no way that i could nail the net as the gutter is made of steel and the roof is fiber glass, if not mistaken. Can anyone give any suggestions?

Stuff rockwool into the holes. It’s fireproof.

Good suggestion, but can the rat munch the wool or maybe make them as nest?

It’s mineral wool / made from rocks so I don’t think so. U can contact rockwool to ask