"If I asked you, would you tell me that this door leads to freedom?"
Assume that the door in question leads to Freedom. If the chosen Guard were an honestant, he would answer 'Yes'. If he were a Swindlecant, he would also answer 'Yes'.
Logic being: The way the question was worded. If the Gringo had asked "Does this door lead to freedom?", the guard would either answer 'Yes' or 'No', but he would have no way of knowing if he was being truthful or not.
Wording it as "If I asked you, would you tell me that this door leads to freedom?" keeps the truthful answer from an Honestant, but it trips up the Swindlecant.
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Scenario 1, in which no conclusion is given; Assume that the chosen Guard is a Swindlecant, since we know that the Honestant answer would be correct in either case.
Gringo: "Does this door lead to freedom?"
(Liar)Guard: No.
This doesn't work out well, because you are left with doubt and are still unable to determine if he is truthful or not, setting Gringo back to square one with no more options.
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Scenario 2, in which the answer is revealed:
Gringo: "If I asked you, would you tell me that this door leads to freedom?"
(Liar)Guard: Yes.
This changes his answer completely- exploiting the fact that he always lies and using it to your advantage.
If the Gringo had asked him "Does this door lead to freedom?", as shown in Scenario 1, the Swindlecant Guard would have said 'No.'
So phrasing it "If I asked you," forces the Swindlecant to answer truthfully in a roundabout way.
No, he would not have told him that the door led to freedom... so he would have to answer 'Yes', that he would have- getting tripped up in his own lie.
If the questioned guard was an Honestant, the answer would be the same, because Yes, he would tell the truth if asked.
Therefor, you can deduce that the door behind the questioned Guard leads to freedom- Swindlecant or Honestant. ;D
The same logic works the other way as well, if the door led to Death- in which case the Gringo would have gone through the other door if he wanted to live.