Is the Gummel-Poon model useful for explaining AC transistor amplification?
You say AC and then immediately involve the capacitors and that's not strictly correct and not also needed.

Power amplification in a transistor means that a change at the input can result in a change at the output and the changes are such that the change (which is a signal) has greater power at the output than at the input. These changes are modeled in the GP model and "work" even when all the capacitors have a value of 0 (zero).

What I describe above is small signal behavior and that has by itself nothing to do with the capacitors. The capacitors are present in the model to model the capacitors that are actually present in the transistor and their effect is that they limit the bandwidth of the transistor circuit. 

At high frequencies the power gain will be smaller or even non-existent (power loss) due to these capacitors.

If what you say is true and the capacitors are involved in power amplification then gain at DC would be impossible. A non-AC coupled common emitter circuit has a DC gain of about gm*Rc.

But back to your question, can the model be used?

Yes of course it can, as long as it models the change in Ic due to a change in Vbe (or Ib) it can. And that behavior is included in the model. If is was not included then the Gummel-Poon model would be "silly" as it would not model the main function of a BJT: power gain.