What force could act upon a body of dispersed gas and initiate the process of causing it condense
Gravity
I can understand perfectly that our sun now holds itself together by gravity. But I would have thought that this force only comes into being when a vast amount of matter is densely packed into an object such as our sun.
You would be wrong, though. Gravity also holds together objects much bigger then stars, like Galaxies
Is it possible that gravity can overcome a gas's attempt to expand even when there is initially no concentrated mass of matter?
ideed it is. But you are right, this is not a trivial question at all. There is the need for some sort of seed (a region that is slightly denser then its neighbourhood). because of that, I gigant cloud of gas can stay stable for very long periods and then go through a period of intense star formation. The point is that once it becomes unstabl, there is no stopping it. the little seeds atract some matter becoming a little bigger, and so forth, until the radiation produced by the newly formed stars dissipates whatever is left of the gas, and the process halts itself
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