To what degree do great people count as buildings? Can they be targeted by things like Gunpowder or Combustion? 

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Sohum said:

Now I don't have the game in front of me but my point of view is as follows: If the text on a tech like Gunpowder or Combustion says it can target a building or wonder for destruction then they can target a building or wonder and nothing else.

That point of view aside, If said techs CAN destroy a Great Person, it would only be moved to the owners reserves, ready to be placed at the next Start of Turn again anyway since Great People never get destroyed. So it would be way more cost-efficient to destroy a building or wonder since you then deny the owner of the building/wonder the trade/construction/Culture until it is rebuilt , in the case of buildings, or forever, in the case of wonders.

They work and are placed just like building but they ain't one.

If something affects building it doesn't work on Wonders and Great People.

There are some Culture Cards that specify that they work on Great People and that GP is lost. The rulebook specifies when can you keep him:

"However, great people have one significant advantage over
buildings. If they are replaced (by having a building, wonder,
or another great person placed in their square), they are held in
reserve and returned to the player’s civilization sheet instead of
being discarded."

So if the city is destroyed or the GP himself is killed, you simply discard him.