The idea of situatedness as indeed a happening that is also a gathering or belonging means that the elements that are gathered together in that happening cannot be understood as elements that have somehow been separated, but are now, in the happening of situatedness, simply returned to their proper relation with one another. Instead, the happening of belonging that is at issue here is indicative of the character of the 'belonging' in question as a matter of the reciprocal determination of elements within a single 'structure.'
That's a quote from Heidegger's Topology, a book I've gotten into over the course of my exam reading this last week.
Essentially - and I'll try to interpret the interpretation -
We start with the idea of situatedness. What does it mean to be situated? Well, if we are going to be situated, we are going to be placed in relation to other things.
Being placed is a happening. We enter into a place, or are put into a place. It happens.
It is also a gathering - because the things in relation to which we are placed are (by being defined in relation to us) gathered to us, or attached to us.
Being in a place, being situated (being in a situation), means being attached.
What this means is that we can't say "to enter a place is to become attached" - because, by being, we are already in a place. To be is to happen. To be is to be attached.
To be is to be attached, and attachment - which is beautifully described by "gathering" - is the creation of structure. To be, then, is to be within a structure of belonging which is tied to a particular place.
That's kind of lovely, isn't it?