What temperatures can a heat gun raise a material up to?
YES a common "garage" type heat gun will heat up the material enough to reshape it. In fact, I know from experience that many household heat guns will heat up plastic enough to melt it, so be very careful.

You say the dent is "large" but with no detail. I would say you could, with a helper, probably un-dent a fairly large area by pusing/shaping while simultaneously heating. But I am sure there are limits so it would depend on your "large".

P.S.-There is no such thing as a "standard heat gun"; they come in all sorts of wattage/heat level configurations.