The master argument of the paper, which Goldberg lays out quite succinctly, is this, which I quote:
P1. The property of being doxastically justified just is that property which turns true unGettiered belief into knowledge.
P2. No property that is internal in the Justification Internalist’s sense is the property which turns true unGettiered belief into knowledge.
C. No property that is internal in the Justification Internalist’s sense is the property of being doxastically justified.
I think internalists have two fairly natural lines of defence. First, one might reject the very notion of some property that turns true unGettiered belief into knowledge, at least if we read 'turns into' in some kind of truth-making sort of way. No doubt there is in some weak sense a property P such that one has knowledge if and only if one has true belief, has P, and is not in a Gettier situation, but I see no reason to suppose that it will be a property any more interesting or natural than the disjunction, knows or false or Gettiered. (I rather suspect "Gettiered" itself can be understood at best conjunctively.) And I don't think there's any interesting sense in which this disjunction turns unGettiered true belief into knowledge.
In defence of this way of setting the issue up, Goldberg writes:
After all, ‘doxastic justification’ is a term of art, and so if we are to continue to use it, it must pick out something that is epistemically interesting. It picks out something epistemically interesting if P1 is true; but it is unclear whether it picks out something interesting if P1 is false. At a minimum, the burden of proof will be on those internalists who deny P1: if this is how they respond to the present argument, then we are owed an explanation of why we should care about the property of which the internalist is purporting to give us an account.
But there are other fairly natural reasons to care about justification available. For example, justification may be that property which permits knowledge, without being one that guarantees it.
The second way an internalist might resist Goldberg's argument is to reject the considerations he brings to bear in favor of his P2. He imagines someone in an evil demon situation who is an intrinsic duplicate of someone with a justified belief. Take her perceptual belief that p. Her belief must be justified, by the internalist's lights, but is not knowledge, since she is in an evil demon scenario. It is not knowledge, even if it happens to be true. This doesn't support the argument unless we can also establish that this is not a Gettier case; at the moment it rather looks like one. (She has misleading evidence for p, and reasonably forms the belief that p on that basis; it turns out that p happens to be true.)
To close off this avenue, Goldberg asks us to suppose that it is probable that our subjects beliefs are true, due to the machinations of the demon.
Still, it is easy to tell yet another variant of the Evil Demon case on which this move – to explain away the ‘no knowledge’ verdict by appeal to Gettierizing luck – is not plausible in the least. Imagine the following scenario, involving the Not-so-Evil Demon: it is just like the ordinary Evil Demon scenario except the Not-so-Evil Demon has conspired to make 65% of your Doppelgänger’s beliefs true (the other 35% being false owing to systematic illusions sustained by Not-so-Evil). Imagine your Doppelgänger in this world. For any perceptual belief (s)he has, there is a 65% chance that the belief is true. If it’s true, this is not merely lucky.
But stipulating facts about luck is a dangerous game. There is of course some sense in which the not-so-evil demon victim isn't merely lucky to believe truly, but is it the one relevant to Gettier cases? Probably not. Nothing in Gettier's original cases precludes probability of true belief of this sort. Go back to Jones and the Ford and Brown in Barcelona; suppose Brown is in Barcelona 65% of the time, and Smith believes that Jones has a Ford or Brown is in Barcelona, as in the original case, solely on the basis of the misleading evidence about the Ford. This is still a paradigmatic Gettier situation, even though there may be some sense in which the belief is true not merely by luck. Given this parallel, I think the internalist has every reason to regard the subject of the not-so-evil demon as in a Gettier case. So there are good grounds for resisting Goldberg's argument.