If an enemy hero calls a heroic action, but it is cancelled due to the Dwimmerlaik's "Sap Fortitude" rule, is Gothmog still able to call a heroic action of the same type for free if he is within 6" using his "Master of Battle" rule?

I don't believe so, as it doesn't take place but the might is still lost.
Having said that, I'm not sure on the exact wording for Gothmog but I strongly suspect that it says "any time" so you could probably get away with it.

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Thanks for the response.
After re-reading the special rules, you don't get the free heroic actions if they are cancelled.
The Dwimmerlaik's rule implies that might is spent before the action is declared, not at the same time, so the action is never called if they choose not to or can't spend the additional might. Gothmog's special rule requires an enemy to be calling a heroic action to take effect.