Personally I would only use 'gravity' with the last one. This is all about collocations, which are words which are used very commonly together. Gravity and situation are two words which collocate very well.
For the middle one I would use 'seriousness of the incident'.
With the top one, an 'event' is usually used to talk about something which has been specifically organised, with guests and everything. So in fact I wouldn't use 'event' at all. I would stick with incident instead.
Hope this helps!