GNSS is global navigation satellite system, "GPS" to most people but the Russians have their own and there is a European one planned. Mavics use multiple systems - hence they see more satellites than a pure GPS system.
MSL is mean sea level , altitudes have to be relative to a reference point, so if you are using a barometer for altitude you zero for the take off point or by setting the pressure to match the landing point. Satellites use a mean sea level as their reference.
KML is a plot file which you can import into mapping software. (GPX does a similar job, there are others).
edit - missed that had already been answered DOH!
If you take the OP's data into a graphing tool (its downloadable from the link he gave and see the reference in
@sar104 's sig) then you can see near the end of the flight the pitch roll and yaw all go berserk, and there's a rapid drop in height (
@sar104 shows attitude data on his chart, I found it made things too hard to read). So we know the drone is falling and tumbling. We can see the lat /lon and altitude at the point where that started, it's roughly on the corner of the building. And we can also see the drone fell from 30.7m above the take off point to 15m below it, so we know we have a height in feet or meters needs to qualified as "above the ground at the building", "above take off", "above msl" etc. (It's like time, if you or I are in England and say 10 O'Clock it's not the same as 10 O'Clock Dallas time where the impact was. We can put everything in Universal time - GMT for old schoolers - but unless the base line is given nobody can be sure what's meant)
We can also see the drone was flying roughly south before impact, initially we thought it hit a wall running east-west but that would mean it couldn't go any further south. It goes south and east after impact so that says it didn't hit square into the building across the street - this is the only thing it could have hit at a suitable angle - so that sanity checks the data. The mini doesn't have on-board collision avoidance, which could have saved the situation.
